I had two olives and they seem to work ok constantly grazing everywhere in the 20g tank. However they don't seem eat much from what I've seen at least to effect the outcome of algae growth. And for the trouble getting them, they died within half a year...so that's it for me. I'll stick with the regular pest snails: pond, ramshorn, trumpet. They are nice to have and cool to look at even if they are more plain. Best way for me to control algae is to have a LOT of plants.