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.
(Thread rework in progress - please pardon our dust!) Freshwater Snail Species In The Hobby (Two Clea helena assassin snails and a Planorbid ramshorn - photo by CassCats) Index Basic Terminology for Identification Physidae: Bladder Snails and Tadpole Snails Lymnaeidae: Pond Snails...
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