I fought hair algae for a long time in my first tank. I tried many different things that did not make a difference. Hermits and snails would get off the algae as fast as possible when I would set them on it. This is what I remember doing to fix the problem.
1. Daily 10% water changes for 10 days.
2. Added macro algae to compete with the hair algae.
3. Mechanically removed the algae a couple of times a week.
4. Tuned the skimmers for maximum output(dark skimmate)
5. Recharged the sand bed
6. Used a turkey baster from the dollar store and blew the detritus out of the algae twice a day. This seemed to make the biggest difference. Once I started that, the algae turned from bright green to brown within a few days
I can't say which of the above fixed the problem or if it was the combination or none at all. Since then I have used different methods to prevent or consume the algae.
Rowaphos or phosban in a phosphate reactor.
Added a long spine seaurchin. It removes all types of algae including coralline and knocks stuff over if it is not attached.
With the hair algae growing, testing for phosphates and nitrate will, in many cases, not show a problem because the algae consumes it as soon as it is produced...
Thanks,
Scott