I've been successfully/unsuccessfully battling hair algae for a long time. My Salifert test kits for PO4, Si, NH2, NH3 show undetectable. Obviously it's there or it wouldn't grow.
I chemically treat with AZ-NO3 (I know, it's a bandaid and not addressing the source). However, with AZ-NO3 it totally eradicates it (hence my statement about successfully battling HA. However, about 5 - 6 weeks later, it starts up again. I run my tests and they are undetectable even when it grows.
I have cheato in my sump, I run carbon 24/7 and I have an oversized Lifereef skimmer. Plenty of snails, crabs and Tangs.
I've resolved myself to the fact, like jh said, that reefs have such an intense biodiversity that can address the algaes overtaking them that we can't possibly recreate that in our glass boxes.
I just try to keep it under control and manageable. Seems like it's an inevitable reef related issue.
master