Targeted hydrogen peroxide works. I used it for a month and there was no rise in nitrates, nitrites or ammonia or any other ill effects people will claim will occur. And it kills GHA very well.
I used 3 ml per 10 gal
total water volume or about 40 ml in my 110+sump. I even carefully used it around my Montipora and killed algae there. I had other issues and the algae came back but hopefully a new robust CUC will take care of that.
Peroxide works immediately and you'll see bubbles for on the algae. Use it when the lights are on as that helps break the peroxide down into water quickly. I just stringed it into the algae and the next day it was white.
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