Good filtration, plants, and algae treatments will help you.Another method is use of ultraviolet light. This application uses uv light to damage single cell algae which may kill some of it, and make others clump together which will allow better filtering from an accompanying biofilter. It's extremely important to run the water through the uv tube at a specific rate that will allow it time to work. Too fast of a flow and the algae may pass without being unaffected.