Scrubbing Black Algae; Do Until Not Visible?


Well-known member
Jun 12, 2019
Phoenix, AZ
Pool Size
Liquid Chlorine
I discovered some small bits of black algae in my gunnite/plaster pool in areas where the paint has been removed (previous owner painted it, stupid).

Question is I bought an 18” stainless brush & wonder do I scrub until the black algae exterior is removed/broken & floats away, then perform the chemical treatment?

I guess what I’m asking is how will I know when I’m done scrubbing, will it be visible as in no more black marks visible or some other indicator?


Texas Splash

Mod Squad
TFP Expert
LifeTime Supporter
Jun 22, 2014
Texas, San Antonio/Marion, South-Central Area
Pool Size
Salt Water Generator
SWG Type
CircuPool RJ-45 Plus
That's a tough call because if you get too aggressive on the spots you may cause unwanted damage to the plaster. Not sure I would go straight to a steel brush in your scenario. Maybe a 50/50 or something else and work my way up in stages if needed? You want to try and break the surface of the BA so chlorine can get to it and hopefully access the roots. In some older pools, you simply can't get to the roots because the surface has become too fragile. I'd say do the best you can based on what the plaster surface will allow, then maintain an elevated FC level for a few days with some consistent brushing. Hopefully the chlorine will penetrate it.

Have a nice weekend. :swim:


Gold Supporter
Sep 13, 2010
Plano, TX
Pool Size
Salt Water Generator
SWG Type
Hayward Aqua Rite (T-15)
How old is the plaster? You don't want to hit very new plaster too hard, and you don't want to hit old weak plaster very hard. You have to just try to scrub where you aren't getting clouds of plaster dust off.

Don't worry about getting it all in one brushing, if that's what you're asking. You are looking to knock the slime caps off so that the chlorine can get at it. It will go away in layers. Once it gets thin enough, then you can get a little more aggressive and get it off of there--but don't try to do it in 1 (or even 2 or 3) brushing. It may take a dozen or more brushings with sustained chlorine levels to get rid of it. I've dealt with it before (& I would say you are never completely rid of it--the roots get down into the plaster and it just waits for your FC to dip).

Personally, after I start making some headway, I found that dosing with Poly60 algaecide is a big help in finally knocking it out (to where I can't see it and doesn't show up in CC and OCLT). I wouldn't bother until you make some headway with it and are habitually brushing multiple times a day (at least morning and night).
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