Correct it's 20mg per gallon in a one time dose. You may have to wait longer for it to kill your gha. It has taken some people around a month or even a second dose depending on how much algae you have. I would wait at least 14-21 days after you dose before doing anything else and see how things are looking.You can run your GFO if you want to help remove some of the extra po4 from the die off.
Ok... Thanks!