Sounds like flat worms to me. A picture would be good to make certain. There is some medicine to can get called Flat worm exit which I have heard works very well. Never used my self but I have heard good things. One thing I have heard about using it is that you need to suck up all of the dieing (sp?) worms or they will release huge toxic bloom, which is very bad. As far as a nature predator I don't believe there a "confirmed" one. Your best bet is to suck them out manually with a turkey baster. Also they don't like light or swift current. Probably cut back on feeding the tank too and turn up your skimmer if you have one. I believe they are a sign of heavy nutrient problems. I always seem to have few but they never get out of hand. Please provide some detail about your tank so we have a better idea with what your dealing with. Other folks will likely chime in tomorrow.