Fish Diseases

Even a novice can spot many of the changes in behavior or appearances that mean a fish is ill. A sick fish may act lethargic or huddle against the edge of a pond, gasp for air, swim frantically and bump the pond edge, or be bullied by other fish. It might clamp its fins against its body or develop red veins or fins, or its dorsal (spine) fin may be droopy.

If you see unusual symptoms, check the water temperature, pH and levels of ammonia and nitrates to determine whether any of these is stressing the fish. Correct any water quality problems and see whether your fish improves.

If fish are clearly ill, it's often best to remove them to an aquarium, where you can treat them with a medicine or salt bath and keep the disease from spreading to the rest of the pond inhabitants. Handle the fish as little as possible. Net each fish underwater and guide it into an underwater bowl or bucket, and then lift out the bowl and lower it into the aquarium water.

Some of the more common fish diseases are:

close-up of a fish with anchor worm attached

Anchor Worm

This isn’t a disease but a tubelike parasite, about 3/4 inch long, which attaches under the fish gill with its hooked head while it produces two eggs at its nether end. The main symptom is a swelling where the parasite is attached. Catch the fish, and remove each anchor worm with tweezers; then treat the spot with a antiseptic such as merbromin or iodine (from a drugstore).
goldfish with dropsy

Dropsy

This ailment, most often caused by bacteria, has a telltale symptom that’s sometimes called the pinecone disease. The fish swells, and its scales bristle away from its sides, making it look like a pinecone. Fish will also get bug-eyed. Dropsy is hard to cure, although a salt-bath treatment may help.
close up with a fish fin with finrot

Finrot or Bodyrot

The fish - often a fantail or shubunkin - develops a white line on its caudal area (where the body joins the tail) and this area becomes inflamed. The tissue disinegrates and needs to be gently removed with a sharp knife; then the fish is put on treatment with a special antibiotic. Antibiotics can be administrered in fish food, in water in an aquarium, or on the affected area directly. It’s always an excellent idea to consult a veterinarian before you administer antibiotics, just to make certain that you’re using the correct medication and dosage.
microscope image of a fluke

Flukes

These are both gill and skin flukes; in either case they are microscopic, parasitic flatworms. When they prey on fish, you’ll see excess mucus plus irritated puffy patches or loss of color, and the fish will act irritated, banging against objects, twiching their fins, and opening and closing their mouths rapidly. A salt bath may help.
fish with fungus growths

Fungus

When fish are stressed and their protective mucous layer breaks down, they become susceptible to fungi normally present in pond water. Instead of tolerating these fungi, the fish sprout cottony growths. Most fish suppliers sell fungicides that can treat the problem.
lice on fish

Lice

These flat, gelatinous creatures are only 1/4 inch long but they have clearly visible legs and eyespots. The tissue damage they cause by attaching themselves to the fish, often on the fins and gills, can eventually prove fatal to the victim. Remove lice deftly with tweezers, and treat the fish with an antiseptic such as iodine.
fish with tuberculosis

Tuberculosis

This is a highlycontagious bacterial disease that can cause rapid weight loss, as the human version does. Fish may lose color and develop raw swollen patches, and fins may waste away. It is almost always fatal. To prevent the spread of this disease, remove any fish from the pond as soon as you suspect it is tubercular.
ulcer

Ulcers

Bacteria common to pond water will cause stressed fish to develop ulcers, popped eyes, bloody patches, and ragged-looking fins. Treat with an antibiotic for fish, and test the water to determine what’s stressing the fish.
fin showing white spot

White Spot

Fish with this disease (technically, ichthyophrthiriasis, or “ich” for short) will have spots like grains of salt, similar to those that males develop on their gills during the mating season. But these will be all over the fish’s body, and because they itch, the fish may bump against the sides of the pond. White spot is caused by the parasite protozoan. Consult with your dealer or veterinarian about a cure.