Nematodes. I had them in my Oscar tank once. Luckily my fiance’s father was the head of the Biology department at JCC and had a Masters in Marine Biology. That’s what he said they were. You’re not going to be able to kill them by adding stuff to the water. You’ve got to do a full water change. Also, you have to either replace everything in the tank or boil all the decorations and stuff, and bake the gravel.