To me it looks like a basic "pond snail" (bladder snail). IME they eat decaying leaves, algae, fish food & anything else they can. In Australia, you may have different snails. Someone will know more. Colin?
You can try smushing them to break their shells. Kind of gross. Or pick out the 1s you see. But look for jelly-like egg masses & remove them as best you can. Feed less, that can help too.