Mystery snails can be very temperamental to water conditions sometimes. Some of mine acclimated to my tank no problem, but other floated in their shell for days. They can and do float at will, for a variety of reasons, but if you don't acclimate and just dump them in and have some floaters don't worry too much (unless they stink as noted obviously).
It's a mixed bag on acclimation needs and techniques, so whatever you are comfortable doing do. When I got my snails they all came in very little water, so there wasn't much acclimation I could do, I didn't have any small containers that the amount of water they had would have been sufficient to cover them anyway. I did leave them in a KK with their bag water and tank water over night (or for my first snail for 2ish weeks) while I finished setting up the tank(s).
Not sure if we can link, but apple snail dot net has good info on them. Research before you get em! The trapdoor is only one of their many interesting attributes :)
Last edited by Quinn; 11-06-2013 at 05:38 PM.