I check my levels once a month, and everything is ok ,even my nitrates are only 20.Why can't I get rid of the red algae. Also I wanted to know how can I clean my coral too get them white. I know this is risky, but the coral gets all brown and red it looks too dirty. Red Algae usually can grow in low light levels. Perhaps your light levels are not intense enough.
Many Red algae are Calcerous type algae.
These algae can encrust rocks, glass, pumps, and virtually any available surface given favorable conditions. I feel it is better to let the algae take their natural course and than to scrape and brush it off all the surface areas. If you are wanting to kill off some of the red algae turn your corals over and turn the substrate over everytime you change the water. If you want a rich green algae growth, you may want to get a good culture from a marine tank that has been established and has a good green algae growth. Sometimes one species of algae gets such a foothold in your aquarium that other types can not compete and grow to a healthy population level.