Like almost all living beings, fish are also attacked by a wide variety of worms. There are worms that infect internal organs as well as worms that only live on the surface of the fish.
Photos: Gerhard Fischer
Pictures of an L 144 and a Congo Tetra from the same aquarium. They may be worms, with different stages developing under the scales. At least the Congo Tetra may have a disease that is called psoriasis on the JBL website.
The different types of worms
Nematodes (roundworms) or their larvae attack different organs depending on the species. Nematodes that require an intermediate host to develop can only reproduce if the intermediate and definitive hosts are maintained in an aquarium. Nematode species that reproduce without a change of host usually represent the greater danger, since they can reproduce easily in the aquarium. Individual nematodes do not pose a threat to the infected fish. However, if they reproduce rapidly and infect the fish in large numbers, the fish often become emaciated and die as soon as one of the affected organs is no longer functional.
Monogenic flukes (monogenic trematodes, monogenea) usually live in the gills and on the skin of infested fish and attach themselves there. Monogenic flukes develop directly from the egg via a larval stage to a sexually mature worm without a change of generation.
Digenetic flukes (digene trematodes, Digenea) undergo an alternation of generations in their development cycle. With one or more host changes, a sexually mature worm develops from the egg via the cercaria (larva with a rudder tail) and the metacercaria (larva without a rudder tail).