I know snails are really great for bettas. I have two nerite snails in my betta's 10 gallon tank. She leaves them alone for the most part!
They are really entertaining to watch, actually. LOL. I have one that is a little smaller than the other and the small one was crawling on the larger one's back. AWW :D They can produce eggs in fresh water,, but the eggs cannot survive unless they are in salt water. :) They eat any algae or leftover fish food. But I have algae wafers in case. BTW My larger snail's shell was a little scraped when I got her..
(Sorry for this one's poor quality, I took in on my ipod since my camera died.)
I got these two at petco for $2.99 ea. They have other types of snails but I liked nerites the most. They also don't get very big- at most 1 inch in diameter. If you decide to get a nerite- your tank would be best for only one nerite.