Are you talking about the large apple snails or snails in general?
I thought that a couple of snails would do to clean the tank but I have ended up with an epidemic of snails. I have a post under tropical fish on page 2. I have received good replies as to how to minimise them.
They may keep a 1 gal tank clean but I dont think them small things can handle much more than that!!!!!!!!!!!!!!!!!!!
Jeezzzzzzzzzzzzzzz those things are huge....wadda you feed em? baby sperm whales?