Tapeworms

Cestodes (Tapeworms). Besides the two kinds of trematodes, tapeworms (cestodes) are the other large group of parasitic flatworms. A wide range of cestodes parasitize fish. A light load of tapeworms is common for wild-caught fish, and fish cultured in open ponds may also carry some tapeworms.
 
The Pseudophyllideans ("imitating a leaf shape") are the group commonest in wild fish. But the Pseudophyllideans are just passing through. They aren't normally an issue with aquarium fish, because they must have a warm-blooded final host in order to mature and spread their eggs. The egg would escape from the fish-eating final host in feces and be consumed by a minute crustacean, such as a copepod. When a fish consumes the infected copepod or worm, the immature cestode travels from the fishes' gut and encysts. End of the line, unless the fish is then eaten by a warm-blooded final host (Once again, no tetra sushi for me, thanks).
 
In other tapeworm families the first intermediate host is an annelid worm. Tapeworms are rare in our aquarium fishes, but since tubificid worms in the wild do harbor some immature life-stages of tapeworms, especially in cool temperate waters, tapeworms figure among our unfounded tubifex fears.
 
Treatment. Praziquantel (marketed as "Droncit"), the de-worming medicine effective in cats and dogs, is coming to the rescue of tropical fish. Until recently you needed a veterinarian to prescribe it for you. Now you can get it on the Internet. The dosage is 2 ppm.
 
The tapeworms that infest freshwater fishes have co-evolved so smoothly with their hosts that they aren't ordinarily very destructive. It's a maladjusted parasite that kills its own host. Tapeworms are hermaphroditic. They do without mouths or intestinal tracts; instead, they absorb nutrition from their rich habitat directly through their body wall. Instead of a mouth, they have various kinds of attachment device, called a scolex. They shed eggs, or even shed entire egg-bearing body segments. Ordinarily there are intermediate hosts, copepods usually, or other invertebrates, as there are with the "di-genetic" trematodes; but a large fish swallowing a small fish may also simply inherit its tapeworms, one of the arguments against purchased feeder fishes and yet another caution against freshwater sushi. Did you know that the longest tapeworm ever found in a fish measured 18 meters! Gulp! how many yards is that? It wasn't discovered in a Dwarf Gourami, I can tell you.