I agree with everyone about using UV. After I installed mine about a year ago my tank has been free from green water algae blooms. I only have a 5 watt unit and it works fine in my 52 gal tank.
Just one important point, UV works based on exposure time. That means if the water passes through the unit too fast, it will not work properly. Usually, you cannot connect the UV to your filter outlet. Typically, you need to install a bypass from your filter outlet to the UV unit to regulate the flow through the sterilizer. This is not hard to do but you need some plumbing to go with the unit.