As I began replacing my gravel yesterday and was taking the old stuff out, a creature popped out of its burial place in the gravel. It was a flat worm-like thing, all brown, no discernible head features, about .75" in length, swam like an eel. It quickly buried back into some other gravel. Not much later I saw another one. What is this, and more importantly, is it good or bad? Thanks for any advice, sorry no pics because it came out so fast and I don't have a digi cam.