More nasty parasites...
The tapeworm Ligula is found in freshwater fish. It starts life as a tiny egg which is eaten by a freshwater crustacean, Cyclops. This is eaten by a fish, and the larva moves into its body cavity (not the gut) and grows into a worm. It grows very large, making the fish fatter and fatter. The resulting fat and slow-moving fish is likely to get eaten by a predatory bird, where the worm finishes its life cycle. The bird's droppings will contain the eggs, which are then eaten by Cyclops. If an affected fish is not eaten, the fish will continue growing normally, and the worm will eventually shrink away. Fortunately not a risk to humans.
Some parasitic flukes in freshwater fish live in the lens of the eyes, making the fish go blind. Again, this makes it more likely to be eaten by a predatory bird. In captivity the fish will survive, but it will never get its sight back.
Parasitic crustaceans attach to the gills and suck blood. Another crustacean, the fish louse (Argulus), swims around freely, attaching itself to passing fish and sucking their blood. It looks like a tiny green stingray, the first time I saw one I thought it was a baby flatfish.
Fish are very unfortunate to have so many parasites.