Types of Parasites on Freshwater Fish

The majority of freshwater fish parasites hobbyists encounter are protozoans, single-celled, free-swimming organisms that frequently do not require a host to reproduce. Protozoans attach to the fish when they are overcrowded or stressed, causing extremely thinness, skin or gill irritation, difficulty breathing, agitation, appetite loss or even death. Protozoans are divided into ciliated and flagellated types, divided by the types of hair-like extensions that aid in their movement. The entire tank must be treated when protozoans are present; it's impossible to treat individual fish for this kind of parasite. Protozoans can be difficult to eliminate from a freshwater tank.

The most common of these protozoans are Ichthyophthirius multifiliis, which cause the disease known as ichthyophthiriasis, also known as “ich” or “white spot disease.” Other common protozoans include tetrahymena, trichodina, Hexamita and Myxozoa.

Trematodes are flatworms found either inside or outside a fish’s body. These flatworms, which attach to a fish's body, are also called “flukes.” Internal flukes are transferred by hosts other than fish, such as snails and related animals. Internal flukes eventually leave their host mollusks and then infest their permanent hosts: your freshwater aquarium fish. By contrast, external flukes are introduced to the host tanks by other fish, such as plecotomus, corydoras and some live-bearing fish, among others.

Gill flukes and yellow grubs are two common types of trematodes. As the name implies, gill flukes attach to the fish’s gills, where they feed on the fish’s mucus, skin and blood. Gill flukes typically have the appearance of tiny, dark spots attached to the gill filaments. These fish can cause fish to become debilitated or even die. Yellow grubs are a large and noticeable trematode that attach to the outside of the fish. Although it does not appear to cause its host any distress, the yellow grub is aesthetically unpleasant.

Nematodes are the same long and cylindrical roundworms that affect animals the world over. These worms infest the intestines and guts of their fish hosts. They're easily distinguished from the shorter, wider appearance of the fluke or the flatter and segmented appearance of tapeworms.

Camallanus is one common nematode that affects live-bearing fish, cichlids and other freshwater aquarium fish. Its red, threadlike appearance frequently causes it to be mistaken for feces when it extends from a fish’s anus.

These parasites attach to the outside of the fish using pinchers, suckers or other formations that allow them to embed themselves in a fish’s gills, body or fins. They are usually readily visible without magnification. Infestations can cause a fish to become debilitated; leeches can cause a fish to become anemic and die.

The anchor worm is a common form of crustacean parasite that affects goldfish and koi during the warmer months of the year. These parasites are large enough that they might carefully be removed by hand. Fish lice are less common in aquarium fish, but they can affect pond-reared goldfish, carp and koi.