Actually, that air bubble you see in your snail's shell is its lung. Yes, mystery snails have a lung and as such, regularly go to the surface for air. Sometimes a mystery snail just likes to sit and float for a while, usually when they've had a large meal and want time to digest it a little.
Other times, it could be that there is a problem with the water parameters. Aquatic snails, like mystery snails, are sensitive to things like high levels of nitrates and ammonia. What are the water parameters? If they are too high, do a 25%-30% water change and then test the water again to make sure the levels have gone down. If not, repeat and check again until the levels are safe.
Also, you didn't by chance add any kind of salt to the water, did you? Mystery snails can't tolerate even a trace of salt in their water.
There is one last possibility, and this might not be the case, but how big/old is your mystery snail? As they age, mystery snails become increasingly less active, until eventually they just stop all together and pass away in their sleep. You can usually tell the approximate age by the shell size. The bigger they are, the older they are because they grow more shell as they age.
“The day the power of love overrules the love of power, the world will know peace.”
― Mahatma Gandhi
Last edited by TheCrabbyTabby; 07-18-2012 at 08:17 PM.