I don't know what they are, but I obtained a 75 gal
from a lady with an Oscar and a whole bunch of other fish that shouldn't be housed with an Oscar, but I digress. Anywho, she didn't take the greatest of care of the tank and those things were everywhere in the gravel, in the water, on the underside of the glass canopy. Gross. I sucked them all up with the gravel vac and sent them down the drain. Don't know what they were, but it took me months to get rid of them all. Just when I thought they were all gone I would see more of them. Ugh. Good luck.