by All Creatures Vet Hospital

Preventative Medicine for Pet FishIn terrestrial veterinary practice “preventive medicine” is often erroneously considered synonymous with vaccination and “deworming” schedules. Of course, preventive medicine encompasses much more, including provision of proper nutrition, maintenance of a healthy environment, and management of other disease risk factors. For pet tropical fish, the lack of available vaccines and well worked out chemical prophylaxis regimens greatly increases the importance of the “other” areas of preventive medicine. Preventive medicine should begin before the pet fish owner sets up their tank or pond, and includes many areas considered “Husbandry”.

I. Tank Set-up and Operation

Proper set up of the tank that will house pet fishes is critical to long-term success. For example, tanks set up in southern or western exposure windows may experience severe algal problems possibly along with deleterious temperature fluctuations. Although aquarium heaters can stabilize a tank in cooler environments, fewer systems incorporate the chilling equipment required to keep water temperatures from rising in summer. Relocating these tanks before they are established can avoid numerous problems. Judicious use of blinds or curtains can help if relocation is not an option. Tanks located near radiators or other types of heat exchange outlets can experience similar problems. Remember to assess the air quality in the area projected for the tank and particularly the air quality near any planned remote air pump.

The configuration of a tank can have a great impact on its carrying capacity. Tall tanks with low ratios of surface area to water volume are hard to clean and manage, and can properly accommodate a much smaller biomass than a tank of equal gallonage with a great deal of surface area. Also, many beginners want to start out small and work their way up in tank size. Unfortunately small tanks are much more dynamic and difficult to manage than large tanks. the slightest shift in water quality usually results in rapidly fatal situations. Larger tanks respond more slowly. Larger doses of toxic substances are required to reach toxic levels, which gives the owner more time to observe the problem and react. We advise clients to start out with a tank that holds at least 20 and preferably 30 gallons.

Tanks should also be constructed of materials that won’t be toxic to the fish. All glass aquaria are constructed from glass and high grade silicone rubber. Older tanks or occasionally very decorative tanks may incorporate metals or other materials that can be a source of chronic toxicity. Similarly, very exotic substrates (sands, rocks, decorations) need to be evaluated to make sure they won’t leech toxic substances. This can be an analytical challenge, and may require a skilled geologist or chemist.

II. Water Quality

Certainly the single most important issue in preventative medicine for pet fishes is water quality. Improper initial start up and water cycling of tanks on biofiltration can result in ammonia and nitrite toxicities. In older more established tanks, improper methods of changing water are often the cause of build ups of toxic wastes or contaminants. Clients often erroneously consider “topping up”, or the replacement of water lost to evaporation, as the same thing as a water change. Unfortunately, toxic substances including heavy metals such as copper do not evaporate with the water. Each “topping up” can add more toxicant, and removes none. Slowly the concentrations build up to toxic levels. A classic example would be a long established tank which has been maintained the same way for years. Now fish are dying, either individually or in small groups. They don’t respond to pet store medications and infectious disease signs are not the principle signs. Water change requires that you remove water first then replace water to the original level (0.75% change per day, 20% every two weeks, or 30% change each month will work well in most cases).

1. pH
The ideal pH level of freshwater aquariums is between 6.5 and 7.5.
Marine tropical fishes thrive at a pH of between 8.0 and 8.3.

2. Temperature
The ideal temperature for most freshwater tropical fish is be between 76 and 80 degrees F.

Abnormal Behavioral Patterns

As in terrestrial pets, fish behavior can be a sign of trouble and a cause of trouble. The following is a list of some of the common behavioral patterns displayed by stressed or diseased fish in aquaria.


The most common problem behavior reported in pet fish is undesirable aggression. This aggression is commonly displayed as chasing or fin-nipping. When these behaviors are witnessed, the social structure of the tank should be evaluated and appropriate measure taken to isolate fish.

  1. Chasing
    This is the rapid movement of one fish in close pursuit of another. The dwarf gourami is a territorial species that will commonly chase other fish away from its established niche. The hovering angelfish may actively chase more peaceful species housed in the same environment. These types of situations may cause active fish like guppies and tetra to hide and refuse food, therefore making them more susceptible to opportunistic pathogens in their environment.
  2. Fin-nipping
    Damaged fins and surrounding tissue are potential sites for bacterial infections.


Resting on the bottom is normal behavior for sedentary species and fish that are asleep. Typical fish that are considered sedentary are: plecostomus, polypterus, lungfish, Chinese algae eaters, and some catfish.

Bottom-sitting may be clinically significant if displayed by a normally active species. If one fish is affected, a bacterial or parasitic disease entity should be pursued. If the whole population is affected, possible contamination of the environment with a toxicant should be investigated.


This may be a sign of one-sided blindness (the good eye will always be to the outside of the circle) or one-sided fin damage. Circling typically becomes apparent prior to recognizable noticeable fin damage.

Color change

This may involve a fish becoming blanched (paleness or decreased intensity of the entire body). This is commonly seen in situations stress as in cold shock or low levels of dissolved oxygen. A specific paleness of the lateral line in neon tetras is highly suggestive of infection with the microsporidian Pleistiphora.

An increase in color intensity or the development of new colors may be seen during periods of courtship.


This is described as aimless, unpropelled motion through the water. This is generally thought of as indicative of a moribund(dying) state.


This describes a fish that turns on its side and makes a rapid semicircular swimming motion. These fish will frequently rub on objects in the aquarium as well. Flashing is considered to be a sign of an “itchy” fish. Ectoparasite infestation is the most common cause of this behavior.


Head-standing is when a fish assumes a vertical position in the water with its head down. This is a serious sign, indicating loss of control of equilibrium or buoyancy, and is secondary to gas accumulation in the abdomen or under the skin. This is common in catfish due to the organism Edwardsiella tarda.


This is a swimming pattern in which fish stay relatively in one place in the tank. This is a normal behavior for angelfish, hatchet fish, Siamese fighting fish, and some of the fancy gold fish.


This is the gulping of air at the surface of the water. It is indicative of severe hypoxia. Three normal behaviors that may be confused with piping are:

  1. Air breathers- these fish normally breathe air from the water surface. Lungfish and some eels typically display this behavior.
  2. Leaf fish which are surface dwelling fish may be confused as displaying this behavior.
  3. Bubble-nesters- fish that build nest for their eggs from bubbles they produce are often confused to be piping.

Hypoxic animals typically act very anxious and are not concerned with other things going on around them. Piping may be secondary to low oxygen levels or to gill parasites.


This is a swimming movement in which the fish does not swim in a normal horizontal plane, but assumes an oblique position with the head directed toward the surface. This behavior is very characteristic in tetras infected with the microsporidian Pleistiphora.

External Parasites of Fish

  • Protozoal diseases constitute the most common disease entity for a tank of pet fish. Some protozoans such as Ichthyopthirius (“Ich”) and Cryptocaryon (saltwater “Ich”) have an encysted stage which is resistant to chemotherapeutic treatment. When faced with a protozoal outbreak you must look for a source. This will most commonly be the addition of an unquarantined animal to the aquarium or the presence of a stressor such as overcrowding or poor water quality. Protozoal diseases are best treated with a medicated bath. Fish treated in this manner should be removed from the display aquarium and placed in a hospital tank. The treatment tank should be well aerated and any carbon filtration should be discontinued.
  • Fungal diseases are usually external and are most always secondary to a break in the integrity of the epidermis and associated mucus coating. Common pathogens include Saprolegnia and Fusarium. If the infection is not severe many fish will heal with supportive care. The fungal colony can be gently removed with a cotton swab and the underlying wound may be treated topically with a disinfectant or antibiotic cream.

Internal Parasites

Internal parasites can be clinically significant in aquarium fish.

Metazoan parasites include the skin and gill flukes (monogeneans), cestodes (tapeworms), nematodes, trematodes, and crustacean parasites. With the exception of a severe monogenean skin and gill infestation the presence of these parasites usually does not constitute an emergency. Antemortem fecal examination or a thorough autopsy will diagnose an internal helminth problem.

Bacterial Diseases

Most bacterial pathogens of fishes are gram negative rods and include such genera as Aeromonas, Pseudomonas, Vibrio, and Flexibacter. Infections can be severe and lethal.

Once a diagnosis of bacterial disease has been made or is at least suspected, a treatment plan should be formulated. Larger pet fishes may be injected intraperitoneally or intramuscularly with antibiotics which are effective against gram negative pathogens.

An alternative to injectable antibiotic therapy is utilizing the oral route. Antibiotics may be mixed into a gelatinized food or given by force feeding.

Recipe for Gelatinized Food

  • Take 250 grams of a well-balanced flake food, mix in a blender with 500 milliliters of water. Mix well.
  • To this slurry add 25 milliliters of cod liver oil and 25 milliliters of vegetable oil.
  • Add a can of tuna or spinach baby food (optional step)
  • Blend well. After blending, add in the medication to be used. Blend well.
  • In a separate pan, heat 500 milliliters of water to boiling.
  • Add 60 to 75 grams of powdered unflavored gelatin (8 to 10 normal size packets) to the hot water, and stir until gelatin is dissolved.
  • Allow the gelatin mixture to cool but not set, add the food mixture to the gelatin mixture and stir well.
  • Place the total mixture into plastic bags and place into refrigerator. After one hour, the food can be broken into manageable chunks for your fish to eat.

A third and less desirable approach to chemotherapy is to administer the treatment as a bath. Antibiotics and other compounds can be added directly to the water. This type of treatment is more appropriate for ectoparasite infections. Fish treated in this manner should be removed from the display aquarium and placed in a hospital tank. The treatment tank should be well aerated and any carbon filtration should be discontinued.

Medicating Fish

At the present time there are only 6 compounds (four active ingredients) approved by the Food and Drug Administration (FDA) for use in aquatic species, and use is extremely limited in terms of species, indication and route of administration. Approved products include one ectoparasiticide (formalin), one anesthetic (methane tricaine sulfonate) and two antibiotics (oxytetracycline and a potentiated sulfonamide). Certain products have been designated to be of low regulatory priority by FDA which suggests that, while not approved, their use is being tolerated by the agency.

When a single fish is ill, the fish usually can be placed in a hospital tank for treating. In cases in which many or all fish are affected, especially with a condition like Ichthyopthirius (“Ich”) and Cryptocaryon (saltwater “Ich”) the entire tank should be treated.


During anytime of display tank treatment, carbon filtration should be discontinued because it nullifies the treatment. If the tank contains a viable biological filter, it should be disabled during the treatment to protect nitrifying bacteria. After the treatment, 30% to 50% of the water in the tank should be changed.

Please feel free to contact your veterinarian’s office if you have any questions regarding your tropical fish. They will do their very best to assist you in the proper correction of the problem at hand.

Best of luck and we hope you enjoy your tropical fish for years to come. Please check the numerous literature sources available for more detailed information on tropical fish and their care.

David E. Hammett, DVM
G. Scott Russell, DVM

Supplies Checklist

  • Fish tank
  • Water conditioner
  • Full hood with light
  • pH kit
  • Gravel
  • Fish Foods
  • Heater
  • Decorations
  • Thermometer
  • Stand (optional)
  • Filter with cartridges
  • Books about fresh water tropical fishes