I bought a mystery snail from Petsmart a few days ago and he's been doing well and active. Today he was active and then a few minutes ago I noticed him floating at the top of the tank. He's not "hanging out" or anything and I felt him and he still feels firm but even if I put him at the bottom of the tank he float right back to the top. Could he have died, gotten air in his shell or what is going on? This is odd. I've never owned mystery snails before so I have no clue on this.