"We have a 20 gallon tank with goldfish & live plants residing in it. The tank has been set up for years. Recently, we had a algae bloom (dark green water) that we couldn't control with just water changes, no light, etc. For over a month we were dealing with this problem! You couldn't even see a 1/4" into the tank! I added just ONE does of API's AlgaeFix, and in less then 1 hour the green water was amazingly clear!! The whole family was just amazed, and we were quite happy to actually see our fish again! I will wait three days, repeat, and then do a water change. We couldn't be more pleased with this product."