I bought a bunch of snails 2 weeks ago. For the first week, they hung out at the top of the tank also. And there were a lot of them, it was weird. But, now, they have moved on to doing what I bought them for, cleaning my glass. Seems, every time I buy snails, they do that. And as for my crabs, I have this one rock with a bunch of holes in it, I call it my crab condo, that's where you will find all the small ones, the bigger ones can't fit in the holes any more.