yeah but the xenia needs phosphates to grow fast like that, so does the hair algae, cyano
etc.... You may be phosphate limited now, with available nitrates, but when the nitrates come under control you may find yourself in the opposite position. It is a shame anyway to see someone who does the work have to struggle with nutrient issues. But I with signs like cyano
and fast growing xenia I would make sure to be hitting the phosphates as well as the nitrates like you are going to do, so you don't trade one issue for another. (What could happen is you get the nitrates down to zero, creating a nitrate limited system, and then see readable phosphates). It all has to do with Liebig's Law, it is pretty straightforward and well explained on wikipedia if you are so inclined.