It's a bladder/trapdoor/tadpole/pond snail. It could be a physid or limnaeid species but their behaviour is pretty much the same. Their numbers will get out of control if you let them. The best way to stop them getting out of control is by not over feeding the fish.
More info here, look at the first two species.
Basic Terminology for Identification Anatomy: Foot: the bottom, muscular portion of the animal used for locomotion. Siphon: a proboscis-like appendage appearing on the side of the shell in some species. Mantle: the soft tissue inside the opening of the shell. Spire: the conical part of a...
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