Parasites

Internal parasites of fishes, such as worms and protozoans, are impossible to identify unless the individual is specifically trained in this sort of work. Fishes as a whole show none of the obvious symptoms of bacterial infections and a sign of gradual thinness and wasting away is the only sign of internal parasitism. To treat the parasites as a whole is the only feasible method of control for the average hobbyist.

Interestingly enough, the majority of trematode (worm) infections come from snails (both human and fish infections owe the snail host their thanks for carrying this disease to them.) In order to control the disease all that needs to be done is to remove all snails. This is easily done in the aquarium by removing all the fish and plants and by adding chlorine to the water. Calcium hypochlorite (HTH) and copper sulfate also kill snails. Use plenty of the chemicals and wash the aquarium out thoroughly before using again. Be sure there are no snail eggs on the plants.

Most hobbyists refer to fishes that waste away as having "tuberculosis".

Large Parasites
Only a few parasites visible to the unaided eye ordinarily attack fishes. These include fish lice (Argulus), leeches, anchor worms (Lernaea), and grub worms (Clinostomum). Most of these parasites will be found on pond fish or aquarium fish recently brought in from ponds. They can also be introduced on plants, pond water or poorly cleaned live food. In nature these parasites are rarely serious causes of illness but in confined quarters they can overpower the fish.

Small Parasites
Two flukes are common parasites of aquarium fishes. These are Dactylogyrus (gill flukes) and Gyrodactylus (skin flukes). Both organisms are directly transmitted from one fish to the other and the rate of transmission is partially dependent on crowding. The affected fish will have a shimmying behavior and will be seen scratching themselves against gravel, plants, etc. A strong magnifying glass will show the slender skin flukes on the surface of the fish. Gill flukes usually cannot be seen; at times they may be visible at the edge of the gill cover.

Protozoa
Ichthyopthirium multifiliis is the name of a protozoan parasite which is the cause of the most common fish disease, "Ich". This is a unique infection in that it is usually fatal if untreated, yet is probably the most easily cured of all fish afflictions.