I use DI water and Phosphate is at 0. Not only so, everything else is dead on perfect pt, I still have hair algea growing on rocks.
I know it's my MH that keeps them rolling, but I can't take the light off cux I have corals and anemone that need MH.
I have lots of helpers to control hair algea thou. I have a lawnmower Blenny. He's good. eat lot's of hair algea, but he's too fast, misses spots here and there. And the Turbo Snails, slow.....but effective, clean the rock to bare white rock. And then my Flame Angel, he plucks hair algea really good, but misses spots here and there, also.
So, you might want to go and get some Turbo snail and a lawnmower, that helps