One of my mystery snails has been hanging out inside its shell for more than a week (I just got back from a week-long vacation and he's not any better)... not sure how long he has been in there. I tested if he was dead and he is definitely alive, he comes out sometimes to stretch, but he never really moves. The other two snails have grown to twice his size already, but hi is still a bit small. I did a big water change to see if that would help before vacation, and I usually do them every other week or so on this tank because it's lightly stocked with 3 mystery snails and 6 nerites (no fish, will become my swordtail fry tank eventually). It's a 10g tank, lightly planted with some random stems from the 20g and a few other plants. Any idea what could be wrong with him? Thanks for any input!