I've seen some really nice black mystery snails at my Petsmart and I would like to buy one tomorrow for Phlox's tank. They have various sizes from very small to quite large (about half the size of a ping pong ball). I'd love to get one of the bigger ones but was wondering if it's better to get a smaller one, or does it matter? Also, since my tank is fairly new it doesn't really have any algae in it yet. What should I feed the snail for now? I've seen some kind of algae pellets at the store. Would these be good for the snail?