I found this on liveaquaria.com
Ramshorn Snails originate from lakes, ponds, and swamps. They stay in shallow waters with a lot of vegetation. Two to four inches of open air space above the waterline in the aquarium is required as these snails need open air to breathe. Ramshorn Snails are very peaceful animals and should not be housed with any other animals that would like to make a meal of them.