Staring closely at my tank today I noticed what appear to be tiny white worms swimming in the water. They curl into an 'O' then flip to an 'O' on the other side to swim. They are so tiny (about 1 mm max) and the fish are gobbling them up when they see them. Must be a tasty treat. So. Question. What are they and what do I do about them? There aren't many (but I suppose they are so small and it's a big tank it's hard to know how bad it is). Tank is 250ltrs, fully stocked, heavily planted. My nitrates are typically on the higher end of the scale (between 40-80ppm).