What should I feed my snail, it ate all my plant?

Hello Out there!

I don not know where this mail leads to, but I am looking for any answer to my small problem.  I appreciate if you could advise me this time or let me know the place that I can ask these questions.
			I have a 15 gal. fresh water tropical fish tank. I use Marine Land's Biowheel filter (150 GPH ?) and under gravel filter with power head (I forgot the capacity).  I have sword tails and neon tetras and a plecostomus and a snail which is about 1 inch size.
I had Java plant. One day I found that snails were eating them till roots, not to mention the leaves. Now my questions are

1. What should I have given to the snail for food?

2. I thought a snail can eat algae so that it can clean the tank some how but it doesn't seem to clean it. Am I wrong?

Thank you for your attention.