The clown trigger which was introduced to my display tank on the weekend after a 21 day quarantine period has started acting up. At feeding time it eats a little(maybe 25% of it's usual fill), then starts twitching, jerking, swimming erratically and going into spasms, as if something's eating it from the inside. It's quite disturbing to watch. Are these flukes or some kind of parasite in the fish? Any ideas what it could be and how I can treat it?