OK this may be a silly question, but.......I know all the reasons to have snails in the tank for the puffers, like helping with their beak growth & all, so I have a bunch of snails. The puffers do hunt them & eat them, but I guess my question is, how do it help with their beak growth? They seem just to be eating the actual snail itself (like the slimy part), and leave the snail's shell empty on the bottom. So how does it help?