I actually bought two snails (99 cents each) when I had my 20 gallon. I know realize people will give them to you for free (most will, anyway). Some people like them, some don't. They don't harm, but will eat a little algae. They reproduce fast, but if you like the idea, try it. I actually accidently killed mine by putting aquarium salt (I put a little too much in my 20 gallon, and keep in mind this was a freshwater tank). Salt and snails don't seem to mix. Keep this in mind if you are trying to get rid of them.