I just got two new zebra nerite snails and one tiger nerite snail yesterday. All of the zebra nerites in the store were at the top of the glass in their tank, and many of the zebras were dead. The one I got was under a rock. I figured it was a water quality issue and that they would be rehabilitated in my tank. Plenty of algae for them and good water quality. My zebras are zooming around, but my tiger hardly moves. He moves extremely slowly and even when he's moving I can hardly see his body because it doesn't really stick out of the shell. I noticed a little crack lengthwise a milimeter above his shell opening about 1/4 of the way across. Today I noticed it got longer! Is there anything I can do??? Will it heal on its own or is he going to die? Is this why he's hardly moving and not coming out of his shell when he does move? I need some help