I've found that snails are a blessing.
Snails are freebies with any live plant purchase, no matter how well you think you shake them off. Anyway, it seems that the tanks with the most snails have less of the hard to remove algae (green to black stuff that grows on rocks and glass). (The stringy stuff I remove by hand, and is only prevalent in one of my tanks.....not sure why, perhaps because of the lighting)
I have two types of snails:
(1) cornet (like a triangle?) shaped snails that live in the substrate and will be all over the glass during the night, I don't know if they really eat the algae? I think they break down the waste though.
(2) and then the ram's horn shaped-shell snails -- they love algae and I believe they help keep the water sparkling clear!
When my snails die off, I know there is a problem with the tank, and algae will start becoming a problem. I could be off about how useful they are, but it seems that a tank with the ram horn snails requires less maintenance. Not sure if you want to introduce them to your tank, as they will certainly multiply and I will hand pick them out - the ram horn one's don't multiply nearly as fast as the cornet shaped ones.
Other thing to consider is how much light the tank is getting -- make sure you get the light hooked up to a timer so the tank doesn't get excessive periods of light a day.