I have a red sea nano max 20 gallon saltwater aquarium. On the top of the back of the tank is about 4 inches of plastic that is under water. It is getting coralline algae and I don't like the aesthetics of it. Is there something I can put on it when I do a water change and the water is lower than this point that will remove it?? I have tried a brush and scraper and that hasn't worked. TIA.