I battled HA for 2 years or so. What worked in the end for me was:
- controlled nutrients: reduced feedings, livestock & favor herbivores, stopped adding new CUC, specially snails appeared to die often causing more nutrients, did aggressive skimming, made sure RODI water is pure
- used some phosphate reducing media. For me the bio-pellets worked best. Maybe because I cold not keep up with the maintenance on GFO when I tried that. Or maybe because they work differently by stimulating bacteria instead of bounding the phosphates?!?
- lots of patience. I added no new livestock for about 1 year, I kept pulling HA out & scrubbed the rocks, siphoned algae & dirt, kept the skimmer clean & the bio-pellets tumbling.
The algae kept growing back, until one day I just noticed that it vanished from some rocks. Then in couple months it was mostly gone.