Sounds like bristle worms. Harmless in fresh water. They look like tiny red earth worms and are most common in the filtration system (food source). Under a microscope you can see tiny hairs on the tail used for propulsion.
I vote detritus worms/planeria. They are harmless. Just do some very good gravel vacs and cut back on the feedings. The detritus worms look like blood worms only they are white. Is this what you are seeing?
