trumpet snails eat algea but they breed like crazy. One day you see one, in a month you'll have a hundred!
Once you have snails, it's very hars to get rid of them unless you get a snail-eating fish but I don't know of any community fish that eat trumpet snails cause their shells are so hard!
I tried to boil everything in my tank except the fish and the filter media. Got rid of pretty much everything but I guess some snails were hiding in the filter media and a month later I had lots, right now the tank is infested. At least they hide in the sand so I never really see them during daytime.