First, don't panic. It's a very common problem. It looks to me like you are having a Cyanobacteria breakout. This is temporary if you do the following: it's a bacteria, not a algae, that loves phosphates. You need to be below .1ppm to keep down this growth. Try some phosphate binders like GFO
to absorb the phosphate before the cyano
gets it. Raising flow will also help a lot as it has to settle in place to grow well. It's why it's not on or near your power heads. You may still be feeding a bit too much and not have enough filtration to fully strip the water of all nuisance nutrients.
I personally think chemical treatments and total blackout only provides a temporary solution, it's best to go to the cause of the problem.
Alkalinity? Calcium could be a bit higher. Also get your Mg
checked. Hope that helps.
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