I just bought 2 nearite snails. I let them acclimate for about a half hour and then put them in my tank. One of them went to town, traveling across the gravel to the glass where he is happily eating, but the other one I put on a plant, he crawled down, and seems to be burrowing in the gravel, he's almost covered now. I didn't think nearites did this. Is there something wrong with him? Thanks!