garden snails won't survive in the aquarium, just go to a walmart or something and get an apple snail, i'm no expert on snails but if you get a few snails your going to end up with baby snails eventually so one is enough, unless all the snails you have are either all males or females or if you want baby snails.... if you dont have plants then most of the big snails in your lfs's will be good choices for your tank