how big is you tank becuase the snail that eats the fish is quite big and you might see it it looks like a welk i think but it isn't a welk. i would check around for a crab they sometimes eat fish
Tell us what else is in your tank, this is just a shot in the dark-but I would think any snail large enough to eat fish (what kind of fish??) would be large enough to see and identify.