This article is from the Aquaria: Disease, Algae and Snails FAQ, by Elaine Thompson, Thomas Sasala and George Booth
Ramshorn snails are very common and come in various sizes. Their shape
is as their name suggests. The smaller varieties (under 1 cm) are not
too damaging to a plant tank, although they seem to relish the tender
leaves of the Hygrophila family.
The other type is the dark and light brown striped Columbian Ramshorn
that can grow big as large as 2 inches in diameter. The stripes run
the length of the shell with a pattern of random width light-dark-
light stripes that stays constant throughout the snails life. These
snails are extremely prolific and have a terrific appetite for plants.