Remember when using Purple Up (or any calcium additive) that you actually need some existing coraline algae in the tank to get it to spread. Purple up does not contain any corlaine algae. But with that said, I have/do use it and had good results in my old 24 gallon tank. I am using up a bottle of it now in my new 75 gallon, which has my old rocks from the 24 gallon.