This stuff recently saved my tank. I know my algae problem started because of a high nutrient condition, but once it was there, I think I had enough algae to basically outcompete any of the other sources that would normally remove nutrients from my tank. I think it's telling that as Algaefix wiped out the algae in my tank, my phosphate and nitrate levels didn't budge (and they were both near undetectable). It's just a guess, but I think an established mat of one of the various species of GHA is going to be a better competitor for phosphate and nitrate than media running in a reactor. The reactor (biopellets, GFO, etc.) can only instantaneously treat the water in the reactor, which is a small percentage of the water that's made it to your sump. An established bed of algae in your tank is 'treating' all the water in the display tank (where nutrients usually first become available), and over a much larger surface area. And unlike GFO, it doesn't clog or wear out over time; it's capacity to consume nutrients goes up the more it grows.
I honestly think there's more to problem algaes than just nutrients and lighting. There's certainly more to it out on real reefs.
FWIW, This was after 4 doses. I didn't stop GFO or skimming, but have never used GAC:
before:
After:
Before:
After:
