Need a better description or even better yet pics of the algae. It could be hair algae or staghorn or black brush/black beard algae or even others that may match your description.
What light are you using? How long is the light cycle for? Are you currently dosing a fertilization program/schedule? If so what all is being dosed?
Not to be argumentative but, I don't think phosphate is the issue.
All of my planted tank experience tells me That's not the case. I dose the estimative index. Have for some time now. I know others along with the creator of the system that do as well. The method is to supply non limiting nutrients by way of basically dosing more then needed. I dose nitrate potassium and phosphate every other day. The Other days I dose iron and micro nutrients. Weekly water change.
I can test phosphate with any kit and have it maxed out but no algae issues.
Need more info from the op to determine the cause. The type of algae and the fix.
I wouldn't use any algeacyde in your tank.
