Nematodes pretty much stay put inside the fish. The only time I've seen them come out is with Caprillaria, that come out of the anal opening of discus and then I had some killifish that had gut nematodes that blew right out the stomach and side of the fish (like the movie Alien - yuck).Additionally - are there nematodes/roundworms/other external parasites that this could be - I tried looking it up - did not see any.
Jay