If your screen is only growing little spots of algae here and there, it means the screen is not rough enough. What is happening is that algae is trying to attach and grow all over the screen, but the water washes the algae away. Only a few areas have enough roughness for the algae to hang on in the water flow. So, you need to remove the screen and really really really sand/scrape/rough it up. If it's a clear screen (i.e., acrylic), you should not be able to see through it. If it's a plastic canvas screen, it should feel prickly. For highest results (and highest maintenance)... rug canvas works the best, but it only lasts so long before it comes apart. Then you have to make a new one. Whatever you use, make it as rough as possible.