The fish will eat planaria and it is a good source of protein for them.. Avoid feeding the fish for 2 days or so and they will munch the worms right up.

(The fishstore may have actually been right, for once, they are harmless, as long as they don't wind up being a nematode of some type, in which case they could be harmful to both the fish and yourself. Nematodes can burrow into the fish's muscleclature or YOUR muscleclature and cause cysts. Access to humans is usually through open wounds. However, if the head looks like the pointed end of an EXIT sign's arrow or an arrowhead or whatever, it is probably a planaria.)
PS: You can google image of planaria you can probably better understand what we are describing.
