INCORPORATING PET FISH INTO YOUR
SMALL ANIMAL PRACTICE
Cooperative Extension Service
Institute of Food and Agricultural Sciences
INCORPORATING PET FISH INTO YOUR SMALL ANIMAL PRACTICE1
Ruth Francis-Floyd 2
Fish are the most popular pets in America, yet rarely
benefit from veterinary care. More than 65 million fish are
maintained in households throughout the United States, and
fishowners frequently have other pets, such as dogs, cats,
and birds. The value of individual fish can range from less
than one dollar to many thousands of dollars for show-
quality koi. Home aquarists, like other animal enthusiasts,
often breed and show their pets and participate in special
interest groups including local aquarium societies and
national organizations such as the American Cichlid
Association and the American Killifish Association.
There are several factors that make it advantageous to
consider adding pet fish medicine to your small animal
practice at the present time. Over the past 5 to 10 years,
some veterinary schools have begun to incorporate aquatic
animal medicine into their curriculum. Many recent
graduates have had some level of exposure to fish medicine
as part of their veterinary education, and some individuals
have had one or more intensive classes and/or clinical
rotations in fish medicine. In addition, some veterinary
colleges and national meetings now offer continuing
education opportunities for graduate veterinarians in this
Concurrently, the Food and Drug Administration is
determining which remedies sold for aquarium fish through
the pet trade are appropriate for over-the-counter sales. It
seems likely that some of these products will be removed
from the market over the next few years, which may mean
that aquarists will have to obtain certain medications,
particularly antibiotics, through their local veterinarians. If
veterinarians are going to serve this important component
of the pet trade, the time to start preparing is now.
Unfortunately, there is a feeling in certain segments of the
pet fish industry that veterinary interest in aquarium fish is
purely profit motivated. In reality, it seems likely that it is
related to the diversity of interests of veterinary students
and recent graduates as well as to the maturation of
aquaculture as an animal industry. Nonetheless, it is critical
that veterinarians demonstrate their ability to make a real
contribution to the aquarium trade, and this implies
obtaining a basic level of expertise that will enable them to
identify problems accurately, assess husbandry practices,
and recommend therapy responsibly.
TYPES OF FISH
Although there are several thousand species and varieties of
fish sold through the pet trade, Gratzek and Mathews
(1993) have divided popular freshwater fish into seven
main groups. The Cyprinoids include tiger barbs, zebra
danios, and goldfish. Many fish in this group are peaceful,
hardy, and well suited to a community tank. Most live
bearers are members of the family Poiciliidae, which
includes the guppies, black mollies, swordtails, and platys.
Fish in this group are peaceful, active, and ideal for
beginners. The anal fin of the males is modified to form a
gonodopodium, which is used for internal fertilization. The
female is able to store the sperm for a period of time, and
one breeding may result in more than one "pregnancy." The
gestation period is 4 to 6 weeks, and the live born young
are able to eat a commercial dry diet immediately. Killifish
are members of the family Cyprinodontidae. Although
highly prized because of their bright coloration, they are ill
suited to inexperienced aquarists. Their husbandry require-
ments are demanding, they are short-lived, and their
temperament is often not suitable for a community tank.
The Siluroid catfishes are popular aquarium pets. They are
primarily bottom dwellers and effective scavengers.
Supplemental vitamin C is recommended for this group of
fish and can be easily provided by adding sliced, parboiled
zucchini to the diet. Characins, represented by tetras, are
This information was published May 1995 as Circularll48, a series of the College of Veterinary Medicine, Large Animal Clinical
Services, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611.
2 Ruth Francis-Floyd, associate professor, Department of Fisheries and Aquatic Sciences; University of Florida, Institute of Food and
Agricultural Sciences, Gainesville, FL 32653.
schooling fish and do well with a number of conspecifics in
the same tank with a large swimming area. The Cichlids
are an extremely popular group of fish consisting of species
from the Amazon River as well as the Rift Lakes in Africa.
Some species, such as the angel fish, are suitable for
community tanks, while others, such as the oscar, are
predacious and can only be kept with fish of similar size.
Cichlids are intelligent and colorful, and many species
provide substantial parental care to their offspring, making
them particularly interesting to the hobbyist breeder. The
Anabantoids, and other labyrinth fishes, are specially
equipped to utilize atmospheric oxygen by breathing air
just above the surface of the water. Two popular fish in this
group are the gouramis and the Siamese fighting fish. The
practitioner is encouraged to read several of the excellent
books available on natural history and husbandry of
specific groups of aquarium fish. There are many excep-
tions to the general comments above.
Tropical marine fish and invertebrates are popular and
valuable aquarium pets. With the exception of clown fish,
almost all marine fish are wild caught. Management of a
marine aquarium is challenging as the fish require a stable
environment and nutritional needs may be unknown or
difficult to meet. Disease processes of marine fish are often
poorly understood, and therapeutic options are limited.
SPECIAL EQUIPMENT NEEDED
Most veterinary practices will have most of the equipment
needed for handling fish patients. Basic equipment includes
a light microscope, glass slides, cover slips, scalpel, forceps
and small, curved scissors. Special equipment specifically
for fish includes water-quality testing equipment (Table 1),
a few small aquaria (5 or 10 gallons each) for hospitaliza-
tion or treatment administration, an air pump, plastic
tubing, sponge filters, nets, buckets, a source of dechlori-
nated water or a chemical dechlorinator, and sea salts. An
initial investment of $200 to $400 should be adequate.
Table 1. Suggested water quality equipment appropriate for a veterinary practice providing service to pet fish clientele.
TEST KIT RANGE COSTa
FF-1A Aquaculture Kit $176.00
Alkalinity 0.4-8 gpgb
Ammonia 0-3 mg/L as NH3-N
Carbon dioxide 5-100 mg/L
Chloride 5-400 mg/L
Dissolved oxygen 0.2-20 mg/L
Hardness 1-30 gpgb as CaCO3
Nitrite 0-0.5 mg/L as N02-N
pH 4-10 pH units
Temperature -300 to 120F
Saltwater Master Test Kit $ 35.75
Ammonia 0-3 mg/L
Nitrate 0-50 mg/L
Nitrite 0-0.75 mg/L
Chlorine $ 32.50
Free and total chlorine 0-3.5 mg/L
Copper $ 27.50
Free copper (low range) 0-2.5 mg/L
prices listed are from Hach Company, 1993-4 catalog, Loveland, CO. Similar products are available from other sources.
bl gpg = 17 mg/1 or 17 ppm
Table 2. Key questions for taking a history for a pet fish
Questions about the fish:
What species is it?
How old is it?
How long have you had it?
How long has it been sick?
Is it eating?
Questions about the environment:
What size is the tank?
How long has it been set up?
How many fish are in it? What species? What size?
What type of filtration is on the tank?
How often is water changed?
Are water quality parameters known?
Questions about the problem:
When was the problem first noticed?
How many fish are affected? What species?
Have there been any recent introductions to the tank?
Have the fish been treated with any medications?
If fish have been medicated, with what? How much?
CLINICAL EVALUATION OF PET FISH
Initial assessment of the fish patient will usually be made
while the history is taken (Table 2). This may be done over
the phone or in the office. Key information should include
the species of fish involved, the number of fish involved,
and the period of time the problem has been in progress.
The size of the aquarium or pond should be determined,
along with stocking density, feeding rate, how long the
aquarium or pond has been set up, how long affected fish
have been in the system, and when the most recent intro-
duction of new fish occurred. Finally, it is important to ask
whether or not fish have been medicated recently and, if so,
with what. Questions regarding product use are appropriate
and provide a means of determining whether the dosage
administered was appropriate.
Once it has been determined that the fish (one or more
patients) is to be brought into the clinic, the owner must be
instructed on how to submit a sample. A water sample is
extremely important and can be collected in any clean glass
jar. Although only about 100 milliliters are needed for
routine analysis, it is a good idea to recommend that the
client fill a clean quart jar to assure that plenty of water is
available in case some gets spilled or additional tests are
required. Water samples should be free of air pockets and
submitted, with a separate fish sample, on ice. Water that
the fish is transported in is not representative of water in
A live fish can be transported in a bucket or cooler partially
filled with water. Aeration with a battery-powered pump is
recommended if the fish is to be in transit longer than 30 to
60 minutes. If the fish is to be shipped from any distance, it
can be live-packed into an oxygen-filled bag (with 30%
water) and then placed in an insulated (e.g., styrofoam)
container and shipped by bus or by air to the clinic. Many
people use overnight mail to move fish rapidly around the
country. A dead fish should be wrapped in a wet paper
towel and transported on ice. A fish that has been dead for
more than 15 to 30 minutes prior to being placed on ice is
not suitable for examination.
ASSESSMENT OF HUSBANDRY
The most important aspect of fish health management is
maintenance of proper water quality. There is a great deal
of variation in the tolerance of different groups of fish;
however, some general guidelines are provided in Table 3.
The most common water quality problem in home aquaria
usually involves accumulation of nitrogenous waste,
particularly ammonia. This is often caused by overfeeding
or inadequate biofiltration. Biofilter failure is commonly
caused by inadequate flow, which may be caused by a dirty
filter. Biological degradation of ammonia (Figure 1) is an
aerobic bacterial process, and a filter that becomes clogged
by accumulation of organic debris often develops anaerobic
areas, resulting in poor performance. Ammonia toxicity can
be acute or chronic, but in either case it becomes more
serious as pH and temperature rise. Diagnosis of ammonia
Figure 1. The ammonia cycle. Ammonia is metabolized by bacteria to
nitrite and then nitrate. This biological degradation of nitrogen by-
products is an aerobic process. Common causes of filter failure are
accumulation of organic debris and development of anaerobic areas in the
NH3 & NH4+
S (Total Ammonia Nitrogen)
toxicity can only be done by testing the water. Immediate
treatment involves changing the water; however, it is
important to determine whether equipment failure or
improper husbandry practices contributed to the problem.
Low oxygen can be an important problem in ornamental
ponds, particularly those with heavy algal blooms (visibil-
ity < 18 inches) and marginal aeration. Heavy algal blooms
are often associated with overfeeding and/or overstocking.
A good rule of thumb is one inch of fish per gallon of
water. Feeding should be limited to once or twice a day,
and fish should only be fed what they will eat in 10 minutes
or so. Oxygen problems in home aquaria are rare and
usually are caused by equipment failure (e.g., loss of
electrical power to the home).
Other water quality parameters that can be important are
pH, temperature, total alkalinity, and salinity in seawater
systems. The practitioner is encouraged to pursue supple-
mental reading on water quality and general aquariology to
improve his or her expertise in this area. A reading list is
provided at the end of this article. Failure to have some
understanding of the aquatic environment is one of the
quickest ways for a practitioner to lose his or her credibility
with prospective fish-owner clients. Water quality problems
can cause catastrophic losses. If all fish in a system die
suddenly within a 24 hour period, a water quality problem
is the most likely explanation.
Feeding of pet fish is often limited to products sold through
the pet trade. While many of these products are excellent, it
is important to educate the client on proper food storage
and to purchase products that are fresh. Few fish foods are
labeled with an expiration date, but food products should
probably be discarded after 2 to 3 months. In addition, it is
probably wise to purchase foods from a store that moves a
large amount of product to increase the likelihood of
freshness. Supplemental foods, particularly fresh veg-
etables (zucchini flakes, romaine lettuce, spinach, peas, and
bok choy), are strongly recommended for many fish.
Carnivorous fish should probably be fed some live food
(e.g., brine shrimp, feeder fish, etc.), although there is
always a risk of introducing disease with live foods. An
ideal arrangement for the serious hobbyist would be
maintenance of a culture of livebearing fish (e.g., common
mosquito fish, Gambusia sp.) to minimize the risk of
introducing disease to a valuable collection.
Sanitation is just as important for maintaining healthy fish
as for any other population of animals. Accumulations of
organic matter favor multiplication of opportunistic
pathogens in the environment, as well as contribute to
water quality problems. If tanks are established and not
overfed, they can be cleaned once a week by siphoning
debris (uneaten food, fecal material, etc.) from the bottom
of the tank and changing about 20% of the water. During
the first 6 weeks that a tank is set up, water quality prob-
lems should be anticipated and tested for and more frequent
water changes carried out as indicated by rising ammonia
or nitrite levels. For clients with more than one tank,
particularly breeders or retailers, either each tank should
have its own net and set of equipment or a net dip
(quarternary ammonium compounds work well) should be
used. If nets or equipment are chemically disinfected, they
Table 3. General guidelines on water quality parameters for freshwater and marine aquaria.
PARAMETERa DESIRABLE RANGE
Dissolved oxygen > 5 ppm > 5 ppm
pH 6.0-9.0 8.2-8.3
Total ammonia nitrogen 0 ppm 0 ppm
Nitrite 0 ppm 0 ppm
Chloride >20 ppm NAb
Salinity NAc 33 ppt
Total alkalinity 50-250 ppm > 250 ppm
Total hardness 50-250 ppm > 250 ppm
NA: Not applicable
aTemperature is not listed because temperature requirements of different species are so variable. In general, tropical species,
both freshwater and marine, prefer temperatures in the range of 75F to 80F.
bChloride concentrations are not measured in saltwater systems; instead, salinity levels are monitored.
CSalinity is not generally measured in freshwater systems, although concentrations of 0.02 to 0.5% (0.2 to 5 ppt) are recom-
mended for recirculating systems.
must be rinsed well before being placed into contact with
fish. Tanks and other equipment can be disinfected with
chlorine (10 mg/1) for one hour; however, chlorine residue
must be completely removed before they are placed into
contact with fish. Never use chlorine bleach in a closed
room containing live fish or to disinfect porous surfaces.
Chlorine residue can be removed by using 7.4 ppm sodium
thiosulfate to neutralize each 1 ppm chlorine.
Quarantine is not always popular with aquarists, but intro-
duction of a newly acquired fish directly into an established
aquarium is foolish. Some pet stores are equipped to
quarantine fish for the owner following purchase. This is a
service that could also be offered by veterinary practi-
tioners. The quarantine period should be at least 3 weeks.
During this time the fish should be kept in a small hospital
tank where it is accessible for observation, physical
examination, gill and skin biopsies, fecal examination, and
medication if necessary. A 10-gallon tank with a sponge
filter is ideal for quarantine of small fish. The filter may be
discarded at the end of the quarantine period or can be
thoroughly washed (a household washing machine works
well), dried, and recycled' with ammonium chloride or fish
Examination of Fish
Many fish can be examined with nothing more than manual
restraint. Fish should be placed on a wet surface for
examination. Many fish seem to stop struggling if the eyes
are covered. If the fish is too large to handle safely or will
not settle down, chemical restraint can be provided using
methane tricainesulfonate (MS-222). Finquel (Argent
Chemical, Redmond, WA) is a brand of MS-222 that has
been approved by the Food and Drug Administration for
use in food fish. Induction can be achieved with a concen-
tration of 50 to 100 mg/1, which seems to be safe for most
species. The compound is acidic and should be buffered to
a pH of 7.0 to 7.5, which can be done by adding sodium
bicarbonate (NaHCO3) to saturation. Any anesthetic
solution should be well aerated. Following induction,
anesthesia can usually be maintained with a concentration
of 50 to 75 mg/1 MS-222. While the fish is anesthetized, it
should be monitored for respiratory movement. If the
operculum stops pumping at any time, the fish should be
placed into clean water immediately.
External, physical examination of a fish is done in the same
way as of other animals. Beginning with the head, a fish
should be examined for lesions, body condition, and
finnage. Eyes should be clear, not cloudy or exophthalmic;
gills should be bright red, not swollen, pale, or puffy; and
the mouth should be clean with no lesions, fuzzy areas, or
organisms visible. The best way to evaluate body condition
on most fish is to assess the amount of muscle tissue behind
the head, anterior to the dorsal fin. Weight loss will be most
evident by a thinness behind the head and protrusion of the
backbone. Skin and scales should be bright and intact. Loss
of scales, ulcerations, dullness, or excessive mucus
accumulation may indicate external parasitism or bacterial
infection. Fins should be intact, not tattered, fuzzy, or
Biopsies of Gill, Skin, and Fin
Examination of fresh material collected from gills, skin,
and fins of fish is an important part of the examination
process. The tip of a few gill filaments can be snipped with
a small pair of scissors. Iridectomy scissors are ideal, if
available; however, any small pair of scissors with curved
tips are adequate. Embroidery scissors are particularly well
suited for gill biopsy. A sample of skin mucus and scales
Figure 2. The condition of gill filaments can be assessed with a light microscope: (a) gills demonstrating telangiectasis and clubbing at the tips of filaments,
which are indicative of damage from ammonia; (b) gills demonstrating significant loss of epithelium following treatment with malachite green.
'To "recycle" a sponge filter means to provide a source of nitrogen to nitrifying bacteria so that recolonization occurs as quickly as possible. At least 3
weeks is required to reestablish the population levels needed for effective biofiltration.
can be collected by gently scraping a glass coverslip over
the surface of the fish. If the fish is not going to be sacri-
ficed, be sure to keep the area sampled to a minimum.
Areas of particular interest should be lesions, bumps, or
ulcers. As in other species, the edge of an abnormal area is
most suitable for examination. Fin biopsies can be taken by
gently snipping a section of fin and preparing a wet mount.
Any abnormal or frayed area is best. Wet mounts of gill
filaments, skin mucus, and fin edge can be examined,
without staining, at 100x and 400x with a light microscope.
Parasites can be identified by their movement or shape. A
number of references are available to assist you in learning
to identify different organisms (see Recommended Read-
ing). In addition to identification of organisms, it is
important to assess the number of organisms present and
the general condition of the tissue being examined, particu-
larly the condition of gill filaments (Figure 2).
Intestinal parasitism can be of concern in a number of
popular species of aquarium fish, including many freshwa-
ter cichlids and tropical marine reef fish. A fecal sample
can sometimes be collected by gently squeezing the
abdomen of the fish. If the fish has been anorectic, how-
ever, this may not work well. If the fish is anesthetized to
ease handling, collection of a fecal sample can often be
accomplished during induction as defecation is common at
this time. If the fish is to be necropsied, intestinal contents
can be examined with a light microscope after the abdomi-
nal cavity has been opened. A negative fecal does not mean
that the fish is free of parasites, as some parasites inhabit
the anterior portions of the intestinal tract. However, a
positive result indicates that parasites are present, although
it may be difficult to assess severity of the infestation.
Many breeders will be willing to sacrifice several fish from
a population for complete necropsy in order to obtain a
more accurate diagnosis than could be obtained from
examination of a live fish. It is important to communicate
clearly with the client before the decision to euthanize a
fish is made. Euthanasia can be readily accomplished by
using an overdose of MS-222 or by simply severing the
spinal cord. The principal advantages of complete necropsy
are that it provides a means of obtaining tissue samples for
bacterial or viral culture as well as for histologic examina-
tion. Of these, bacterial cultures are the most important for
routine problems. Bacterial cultures should be taken from
posterior kidney and other organs as indicated. Brain
cultures are particularly important if the fish shows signs of
neurologic disease (i.e., spinning, convulsing, etc.).
Bacterial cultures should be sent to a laboratory familiar
with fish samples. Most bacteria of fish grow best at 25C
rather than 37C and may not key out properly with
identification kits used for mammalian samples.
Bacterial diseases are common in fish; however, in most
cases it is prudent to determine why the fish was suscep-
tible to bacterial invasion. Poor water quality, dirty condi-
tions, and excessive parasitism often predispose fish to
bacterial infections. With the exception of Streptococcus,
which is rare, most bacterial infections of fish are caused
by gram negative organisms. Aeromonas hydrophila is the
most common bacterial pathogen infecting warmwater fish.
The disease often presents with low to moderate levels of
mortality, hemorrhage of fins and skin, and fluid accumula-
tion. Pseudomonasflorescens can mimic A. hydrophila
infections in warmwater fish. A closely related disease that
is common in goldfish and koi carp is Aeromonas
salmonicida, which can cause a chronic, fibrinous peritoni-
tis in some fish. Members of the genus Vibrio are the most
common bacterial pathogens of marine fish, and clinical
presentation is very similar to A. hydrophila infection of
Occasionally, Streptococcus or Edwardsiella ictaluri will
be isolated from the brain of fish showing signs of neuro-
logic disease, often manifest by spinning. Edwardsiella
tarda and P putrifaciens are occasionally isolated from
malodorous ulcers on fish and can cause systemic disease.
Flexibacter columnaris is a bacterium that infects epithelial
surfaces and is a common cause of "fin rot." External
infections of E columnaris can be presumptively identified
with a light microscope. Long flexing rods typical of the
organism can be seen at 400x. Ordal's is a special medium
used for isolation of columnaris bacteria. Bacterial culture
and antibiotic sensitivity testing is extremely important for
pet fish. Trypticase Soy Agar with 5% bovine or ovine
blood is ideal for primary isolation of most bacteria.
Mueller Hinton medium is effective for most sensitivity
Experience at the University of Florida suggests that the
most common infectious disease problems of pet fish are
parasitic. Goldfish and koi, for example, are commonly
presented with heavy infestations of monogenetic trema-
todes on the gills and skin. Monogenes can occur on any
species; however, goldfish, koi, and discus (an Amazon
cichlid) seem highly susceptible. Monogenes are easily
recognized as flatworms visible under low power (40x)
with a light microscope. They move in an inchworm
fashion and attach to host epithelium with hooks.
Ichthyopthirius multifiliis, commonly called "ich," is a
ciliated protozoan that is an obligate parasite of fish. Ich is
very large (1000OOm) and easily seen using low power
(40x). It appears as a large, dark organism covered with
cilia. It has a horseshoe-shaped macronucleus and moves in
a tumbling or ameboid fashion. A similar organism,
Cryptocaryon irritans, causes ich in saltwater fish.
Icthyobodo (previously known as Costia) is a small
flagellate that attaches to the gills or skin of fish. This
parasite is commonly missed by experienced aquarists who
"scope" their own fish. It is difficult to recognize at
magnification less than 400x, but its characteristic cork-
screw-like swimming pattern will suggest its presence to
the careful observer. Once it attaches to gill or skin epithe-
lium, it is difficult to see without oil immersion. The best
place to look for this parasite is on the edge of the gill
filament or skin mucus.
A final parasite worthy of individual mention is Hexamita
(soon to be renamed Spironucleus), an intestinal flagellate
common in the gut of cichlids but also found in other
species. A typical case history includes chronic weight loss,
decreased fecundity, and poor survival of fry. The organism is
easily identified in fecal smears or wet mounts of intestinal tissue
examined under 100x to 400x magnification. The organism, if
problematic, will be present in large numbers, and the intestinal
contents will be "alive" with small, rapidly moving flagellates.
Clinical disease attributed to Hexamita seems most pronounced in
fish maintained under unsanitary conditions or subjected to recent
shipment and handling.
Although several systemic fungal diseases of fish have been
reported, the most common fungal infections are limited to
epithelial surfaces of gill, skin, or fin. In general, most of these are
arbitrarily assigned to the genus Saprolegnia and can be presump-
tively identified by the presence of broad, aseptate hyphae from
suspect lesions. Should fungal cultures be desired, Sabaroud-
Dextrose medium is a good choice for initial isolation. Most
external fungal infections are secondary to husbandry problems,
particularly overcrowding and poor sanitation.
Although there are some viral diseases of tropical fish, most are
poorly understood, and diagnosis is based on "ruling out" other
problems. Lymphocystis is caused by an Iridovirus and can infect
many species of fish. It results in blister-like skin lesions caused
by hypertrophy of epithelial cells and can be presumptively
identified by the appearance of giant cells in skin scrapings of
Recently, an Iridovirus has been associated with systemic disease
in gouramis. Affected populations can suffer significant mortality
A Paramyxovirus was associated with systemic disease and
heavy mortality (>50%) of angelfish in 1990. Infected fish
huddled in comers, clamped their fins together, and often
developed severe tail rot in the absence of obvious parasites or
A Retrovirus has been associated with the presence of lip
fibromas in angel fish (Figure 3); but affected fish were not
systemically ill, and surgical removal of the mass was effective.
Figure 3. Lip fibromas in angel fish Pterophyllum scalare. Presence of
these lesions has been associated with retrovirus-like particles, identified
by transmission electron microscopy. Clinical management by surgical
removal of lesions has been successful.
TREATMENT OF COMMON DISEASES
Treatment of External Parasitic, Bacterial,
and Fungal Infections
Most external parasite infections of fish can be controlled
with one of the following compounds: salt, formalin,
potassium permanganate (KMnO4), or copper sulfate
(CuSO4). All are similarly effective, although potassium
permanganate may be a little more efficacious against
Saprolegnia and columnaris bacteria (external infections
Although not specifically approved for aquaculture use, salt
has been designated as a "low regulatory priority" com-
pound by the FDA, and its use is widespread in most
aquaculture industries. It can be added to freshwater
recirculating systems or small ornamental ponds, as a
permanent treatment, at concentrations of 0.02 to 0.1%,
depending on the species of fish. At this concentration,
most chronic problems caused by protozoans will be
eliminated. When receiving a new group of fish, a 3% salt
dip will effectively eliminate many external parasites from
freshwater fish, whereas a freshwater dip will similarly
benefit marine fish.
Formalin is FDA approved for control of external parasites
on channel catfish, salmonids, and paneid shrimp. It is
effective against monogenes and many protozoans.
Formalin can be added to aquaria at a concentration of 25
mg/L for a long-term bath, which is approximately 1 ml per
10 gallons of water. Short-term baths of 250 mg/L for 30 to
60 minutes can be used to treat fish in the clinic. However,
treatment of a sick animal can result in its death, so be
cautious and never leave a fish unattended during treat-
ment. Formalin is algicidal, and each 5 mg/L chemically
removes 1 mg/L dissolved oxygen from water, so aeration
is mandatory during treatment. For this reason, formalin is
not recommended for use in ornamental ponds with
marginal aeration and/or heavy algal blooms.
Potassium permanganate is not FDA approved, but has
been used for many years by aquaculturists as an oxidizing
agent. It effectively eliminates many parasites as well as
bacteria and fungi from the external surface of fish. It is
caustic to epithelial surfaces, however, and should never be
used more than once a week. Potassium permanganate
should be delivered at a concentration of 2 mg/L as a
prolonged bath. Delivery of the chemical to aquarium
systems is best accomplished by using a stock solution. For
KMnO4, a stock solution that will deliver the desired
concentration (2 mg/L) can be prepared by dissolving 285 g
of the chemical into 1 gallon of distilled water and dispens-
ing a dose of 2 drops per gallon. This concentration should
change the water to a light pink color. If the water color
changes from pink to brown, or clears, in less than 4 to 6
hours, the treatment may not have been effective. Potas-
sium permanganate is quickly deactivated in water that has
a heavy organic load. To compensate for this, the treatment
can be repeated, in 2 mg/L increments, until a concentra-
tion of 6 mg/L has been achieved. If it is still not possible
to maintain a pink color for 4 hours (from the time of the
first application of KMnO4), the system should be cleaned
and the problem reevaluated. A short-term bath of KMnO4
can be delivered by using 10 mg/L for 30 minutes.
Copper sulfate is extremely effective against protozoans
and is inexpensive. Although not FDA approved for this
purpose, it is EPA approved as an algicide and has been
used extensively in commercial aquaculture for many
years. Copper sulfate is very toxic to fish and so must be
used carefully in a system containing live fish. For freshwa-
ter systems, a safe way of achieving this is to titrate the
concentration of CuSO4 based on the total alkalinity of the
water. Total alkalinity (TA) can be quickly measured with a
test included in the Hach kit (#FF-1A). If TA is less than 50
mg/L, CuSO4 is contraindicated. If TA is between 50 and
250 mg/L, the concentration of CuSO4 should equal TA/
100. For example, if TA is 100 mg/L, then the desired
concentration of CuSO4 is 100/100 = 1 mg/L. If TA is
greater than 250 mg/L, the concentration of CuSO4
delivered should not exceed 2.5 mg/L. A stock solution can
be made by dissolving 285 g CuSO4 into 1 gallon of
distilled water. The stock solution mentioned above will
deliver 1 mg/L CuSO4 for every drop per gallon dispensed.
Copper sulfate must be used with caution in ponds because
it is a powerful algicide and can cause a catastrophic
oxygen depletion. Aeration must be provided when treating
fish with any chemical.
Determining how much chemical to add to a system to
achieve a specific concentration is fairly straightforward as
long as the volume of the system is known. Most aquarists
determine volume in gallons, and 0.0038 grams of any-
thing, dissolved in one gallon of water, is equal to 1 mg/L.
Table 4 gives several examples of how this information can
be used to calculate the amount of chemical to deliver in an
Table 4. Examples of calculations used to determine amount of chemical required to deliver a specific concentration of
compound to an aquarium system.
To treat a 125 gallon freshwater aquarium:
Chemical Desired Concentration Formula
Formalin 25 mg/L. (2 drops/gal.) 125 gal X 2 drops/gal =
250 drops (12.5 ml)
Potassium 2 mg/L 125 gal X 2 mg/L X 0.0038 =
Permanganate 0.95 gram
Copper Sulfate Total alkalinity/100, 125 gal X 220/100 X 0.0038 =
if tot. alk. = 50-250 mg/L. 1.0 gram
(Assume tot. alk. is 220 mg/L.)
Masoten 80% 0.25 mg/L. (For a product 125 gal X 0.25 mg LX 0.0038X
wettable powder that is only 80% active, 1.25 = 0.15 gram
use 100/%AI= 100/80= 1.25.)
Salt 0.2% = 2 ppt = 2000 ppm = 125 gal X 2000 mg/L X 0.0038 =
2000 mg/L 950 grams = 2.1 lbs
Treatment of Systemic Bacterial Infections
Use of antibiotics in fish should be based on results of
sensitivity testing whenever possible. Since most bacterial
infections of fish are gram negative, oxytetracycline is a
good broad-spectrum antibiotic that can be delivered in
feed, by injection, or in a bath. Baths should only be used
when fish are not feeding and injection is not practical.
Bath treatments should deliver no less than 750 mg
oxytetracycline per 10 gallons of water and must be
repeated daily for 10 days. Oral medications should provide
55 mg oxytetracycline per kg body weight for 10 days.
Because pet fish patients are so small, it is often necessary
to estimate the amount of antibiotic to add to the feed. One
gram active drug per pound of feed is a good rule of thumb
for many antibiotics. Commercial flake diets are available
that contain Terramycin, a product containing
oxytetracycline that is FDA approved for use in catfish and
Romet is a potentiated sulfonamide containing ormetoprim
and sulfadimethoxine. It is FDA approved for use in catfish
and salmonids and is available in a flake food for tropical
fish. The desired dose is 50 mg active drug per kg body
weight for 5 days. Romet is not water soluble, so it is not
suitable for bath treatments and is not available in an
Erythromycin delivered in feed at a dose of 100 mg/kg for
14 days is the treatment of choice for Streptococcus
infections. At the present time, erythromycin is not labeled
for aquaculture use.
Treatment of Internal Parasitism
Metronidazole is the treatment of choice for Hexamita
infections. It can be delivered orally at a dose of 50 mg/kg
(approximately 4.5 g per pound of food) for 5 consecutive
days. There is a commercially available flake food that
contains metronidazole. If fish are anorectic, metronidazole
can be provided as a bath delivered at a concentration of 5
mg/L (approximately 250 mg per 10 gallons) daily for 5
Although tapeworms have not been discussed, the practitio-
ner should be aware that praziquantel is extremely effective
in fish and can be delivered as a bath (10 mg/L for 3 hours)
or in a medicated food (100 mg/25 g food, daily for 7
days). Praziquantel is also effective against monogenetic
trematodes, and there are anecdotal reports that it has some
efficacy against digenetic trematodes as well.
Fenbendazole can be used to control enteric nematodes,
although complete information on safety and efficacy is
lacking. Angel fish in our laboratory that were treated with
11 mg/kg orally for 2 days had some benefit from
fenbendazole; however, efficacy of a 3 to 5 day treatment
regime should be evaluated.
Pet fish medicine provides a unique opportunity for small animal practitioners to build their practices by serving a group of
clients who have historically been neglected. An increase in demand for veterinary service is anticipated as the aquarium
hobby matures and realizes the benefits of accurate diagnosis, responsible therapy, and preventive medicine programs.
Veterinarians willing to provide this service must take it upon themselves to attain a level of expertise that will allow them to
make substantial contributions to the well-being of their clients' animals and businesses.
Brown, L.A. (Ed). 1993. Aquaculture for Veterinarians: Fish Husbandry and Medicine. Pergamon Press, Oxford. 447 pp.
Gratzek, J.B., and Mathews, J.R. (Eds). 1993. Aquariology: The Science of Fish Health Management. Tetra Press, Morris Plains,
NJ. 330 pp.
Post, G. 1987. Textbook of Fish Health, Second Edition. TFH Publications, Neptune City, NJ. 288 pp.
Roberts, R.J. (Ed.). 1989. Fish Pathology, Second Edition. Bailliere Tindall, London. 467 pp.
Stoskopf, M.K. (Ed). 1993. Fish Medicine. W.B. Saunders Company, Philadelphia. 882 pp.
Stoskopf, M.K. (Ed.). 1988. "Tropical Fish Medicine." Veterinary Clinics of North America: SmallAnimal Practice 18(2):474.
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