The spike-topped apple snail, Pomacea bridgesii.
Just be sure it's a real bridgesii. (See
www.applesnail.net) The LFS, even
very good ones, generally won't know the different between one apple snail and
another. And the other Pomacea species eat live, healthy plants.
Leigh
http://www.fortunecity.com/lavender/halloween/881/