Preventing disease is always preferable to treating it. In most cases, a comprehensive fish health management program should focus on water quality, nutrition, sanitation (maintenance and cleaning), and quarantine.
Fish show signs of illness in a variety of ways. Some general signs of illness include changes in swimming behavior and noticeable changes in the color or condition of the body and fins (see Common Signs of Illness in Fish). The sections of this discussion that cover individual body systems also list more specific signs as they relate to certain diseases.
You will not be able to give your sick fish a pill or a spoonful of medicine. However, it is possible to give medications to fish to treat various disorders. Methods used to administer medication to pet fish include topical application, injection, immersion, and administration in food. Some of these (such as injections or surgery) need to be performed by your veterinarian, whereas others can be done at home. For example, many ulcerations and sores can be treated with a topical ointment. A number of scale, skin, and gill problems, particularly parasites, can be treated by placing the fish in a tank of medicated water for a period of time or by adding medication to the main tank or pond. Some freshwater fish can be treated by temporary immersion in mildly salty water, and saltwater fish can sometimes be treated by immersion in fresh or less-salty water. A few internal problems can also be treated by immersion methods. Simple internal problems can often be treated by feeding medicated food. More complicated diseases require veterinary treatment.
Certain drugs have been approved by the US Food and Drug Administration (FDA) for use in food fish, while others are listed as “low regulatory concern.” These drugs and compounds can be used to treat pet fish. Because there are relatively few drugs approved for use in fish, drugs and compounds that have not been approved by the FDA are also sometimes used by veterinarians under controlled conditions. Federal and state regulations are of concern when treating outdoor ponds because contamination of the groundwater and other animals can be an issue.
The best preventive care to keep fish and their environment healthy includes providing a good diet, monitoring the water quality, and maintaining a regular schedule of cleanings, water changes, and filter replacements (see Essential Maintenance). Cleanings should include stirring up the gravel or other substrate to release solid wastes that have been trapped. The wastes can then be removed by the filtration units or by use of a net. A pressurized water and vacuum system can be used to remove solid waste from the gravel while water is being removed for a water change. Any new water should be dechlorinated before it is added to the aquarium or pond. If needed, salt and other supplements should be added to the new water.
Decorative objects should be cleaned every so often. Crevices, cracks, and holes on rocks, wood, decorative corals, and other objects can capture waste that can pollute the water. Remove the decorations from the aquarium and run them under hot water. Rubbing or scrubbing with a towel or clean brush may be needed for more porous or obviously dirty pieces. Let the pieces drain completely until they are no longer dripping. They can then be returned to the aquarium without much worry of harm from chlorine.
All filtration systems, regardless of type (including trickle filters, canister filters, sand filters, sump filters, ultraviolet sterilizers, and others) need regular maintenance. Replaceable cartridges should be changed once a month, or more often if the filter floss becomes clogged with wastes and the activated carbon loses effectiveness.
Quarantine means keeping new or sick animals in a separate aquarium for a specific amount of time before joining others. The purpose of quarantine is to prevent the accidental introduction or spread of infectious disease to an established population of aquarium fish. Valuable pet fish should be quarantined for at least 30 to 60 days before being added to the general population. This can prevent the spread of disease from the new (or ill) fish to the entire population. A quarantine tank can be set up for new fish or for those showing signs of disease. Isolating the fish in question also allows easier observation and treatment. Quarantine systems should be completely broken down and disinfected between uses. Once quarantine is started, no new fish can be added or the quarantine period must be restarted.
In large-scale fish operations such as salmon hatcheries, vaccination is quite effective and commonly done. Vaccinations are still uncommon for pet fish, but it is likely that more vaccines will become available, such as the one that can prevent ulcer disease, a serious infection caused by Aeromonas salmonicida, Aeromonas hydrophila, or other bacteria in koi and goldfish. The vaccine can be given either by injection or by immersion, with the size of the fish determining the method of administration. Routine vaccination of koi or goldfish is not recommended.
For all vaccines, water temperature affects the speed of the immune response, or how fast the body builds up antibodies to the disease. An experienced veterinarian or hobbyist can advise you on how to properly administer vaccines to fish.