Black Algae persistant

[Phillbo]

Drain it , spray it with pure bleach and let it dry out?

[CarlD]

I'm no expert on hard surface pools but the power washing may actually be abrading the surface allowing MORE nooks and crannies for the algae to bed in.

But what I didn't see in your post was your raising your FC level extremely high and keeping it high (Like, say 50 or 60ppm) to attack the algae. You should also consider raising your borate level to 50ppm (use Borax to raise borates, and muriatic acid to adjust pH down) as it inhibits algae.

Meanwhile, please provide us with detailed test results as well as the gallonage of your pool.