I once purchased an "in-use" canister filter from a LPS that was going out of business. In doing so I inadvertantly introduced a colony of tiny snails about the size of mosquitos. They kept my Managuense tank spotless for over 6 years. they were too small to bother my fish and when I could see them all over I simply put a piece of spinach in the tank tied to a string and the next morning pulled a bunch out and thinned the group.