I had a similar problem for a couple weeks if not longer. They were small white/silvery like wroms that would float around in the current or hang out at the bottom (bare bottom growout tank). To get rid of them, start with some salt, then do about 3 water changes a week, syphoning out as many as you can, and also cut down on feeding. I did this for a little over a month, and now I havnt seen a single worm in a couple weeks. They could very well be harmless, but they definatly arent pretty to look at either.