Parasites pose a very real risk to to fish All fish are potential hosts to many different species of parasites. Small numbers of parasites are common and probably do little harm. However, all parasites have tremendous reproductive potential and can, under ideal conditions, quickly overwhelm fish in the confines of a tank or pond.
We can broadly class parasites into two groups.
Ectoparasites - are found on the external surfaces such as skin, fins and gills
Endoparasites - are uncommon in ornamental fish.
However, there is a wide diversity of blood parasites, worms that colonize the intestines and other parasites that can invade various organs and tissues. Many require an intermediate host, such as snails, birds or the introduction of an infected fish, and so are rare in pond/ aquarium fish. There are a variety of clinical signs which might indicate the presence of endoparasites. Lethargy accompanied by emaciation is a common sign, as are worms protruding from the anus. Identification of sporozoan and protozoan endoparasites often requires microscopic examination of tissues.
