why is my snail floating?

It's a mystery! Jk, mine do that too once in a while so i'm pretty sure it's normal.

That’s very common as noted, and is really just due to the snail getting air trapped inside of the shell (that will come out naturally or you can turn the snail upside down to remove the air sometimes).

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As long as it keeps moving at some point, it sounds fine! Are they going near the bubbles from the pump maybe? Perhaps they enjoy the float?