A snail seems like the best choice to me. Keep in mind that mystery snails get rather large and are thus big waste-producers. A nerite snail might be a better choice. I have one in my 72 gallon right now, but I used to keep it in a 10 gallon, and it cleaned it rather well. In that tank, before I got the nerite, there was a rather large (dollar bill-sized) patch of algae growing on the filter side of the glass. The nerite snail had it cleaned up within 2 days.