What can be done about these worms I have in my tanks? The majority of the ones I see are at the top of the aquarium between the top of the tank and the water line. They are about 2-3 cm in length there and very small in diameter. I usually scrape them off and the fish seem to love to eat them. If i cut them in half they seem to divide into 2 living worms.. If i stir up the substrate there can usually be a few found swimming around as well.