For the last few nights we have spotted one or two worms, half an inch long swimming around our tank. They come out of a rock, swim to the surface, do a couple of laps and then go back to a rock... but one night before a worm returned to a rock our sailfin tang ate it. The tang was pretty sick lying on his side on the bottom as if paralysed for all of five minutes then fine again. They are white and swim like a sea snake, what are they and how do I get rid of them? Thanks