Hi Mack;

Pressure washing is one of the most effective ways to remove visible black algae . . . but it won't remove all of it. And, it WILL come back, if you let your pool have readings like you reported (FC=0).

If you want to avoid a recurrence of *visible* black algae, you have to plan to avoid a recurrence of "FC=0" or even "FC=1 or 2". Simply put, once you've had it in a plaster pool, you should assume (usually correctly) that it is STILL there and will begin re-growing EVERY time you let the chlorine get low.

You need to read and understand the BBB page (linked in my signature) if you want to keep your black algae invisible.