otos eat brown algae and soft green slime algae. they will not touch hair, red, black, staghorn, brush, beard, etc. i have 3 otos in my 20g tank and they have kept it totally free of brown and green slime algae, but i am now fighting hair algae and they are no help at all. however, i love them anyway because they are just adorable, funny, cute little fishes all on their own.