Because there are nutrients in the water and no competitors to remove these nutrients before they are consumed by cyano or algae. Who are these competitors and where are they? Bacteria. As it happens, your skimmer/UV/ozonizer destroys bacteria in the water, but not the cyanobacteria and algae attached to the aquarium walls. The long-term solution to the cyanobacteria/algae problem is to add bacteria to your system that can consume nutrients and then remove these organisms, thus eliminating nutrients from the system. We recommend using our Cyanobacteria program