It's a pond/tadpole/bladder snail, one of the pest species. The photo shows a Physid snail. These usually arrive on live plants. Despite what other members may say, they are not bad; they are actually a useful part of a tank's ecosystem - provided they don't take over. The easiest way to keep their numbers under control is by not over feeding the fish. Uneaten food = more snails. And keep the tank bottom free of other edible bits (eg bits of plant). I have these and those tiny ramshorn snails in both my tanks and I am careful not to over feed my fish. They need less food than you'd think.