A lot of people who are having hair algae issues have low calcium, alkalinity, and magnesium. The latter two being more significant. Saying your cal and alk are "high", and your mg is "normal" doesn't say anything...what are they actually at?
A prime example...I noticed the tiniest of shoots of hair algae in my tank, so I tested my parameters which I haven't done in awhile. Hehe...oops. My magnesium was down around 1250 ppm, my alkalinity was just over 7 dKH, and my calcium was down at 370. I brought my mg back up to 1400, and my alk back to 9-10, and my calcium to 430, and poof hair algae gone. I didn't do anything else to the tank. My tank hasn't had hair algae since it was first cycling over a year ago, so I knew something was up.
"Everyone" knows that nitrates and phosphates are significant contributors to algaes in general, but keeping
all your parameters in check is important even in FO tanks.
