sounds like an nutrient spike causing the algae bloom. I had something similar when I reset my tank this past fall. All sorts of funky algaes, cyano, etc popping up where it never used to before. Then I decided to put my BioPellet reactor back on line to more aggressively attack nitrates even though it was already pretty low at <4ppm (certainly not Ultra Low, but I've never had algae problems with that kind of a nitrate reading in the past). Anyway, a few weeks of that and the algae started to die back. I also had to change GFO more frequently (like once a week!) because it was getting used up pretty quick. Probably due to phosphates leaching out of my old rocks. I' think I've sucked it all up now cuz I'm back to once a month replacement with the GFO.
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