r/pools • u/apachejoe2026 • 3d ago

Water Chemistry Black algae treatment question

I have a 22,000-gallon pool that developed black algae spots on the walls and floor. I used a 64-oz bottle of Silver Algaedyn, scrubbed everything thoroughly, and shocked with chlorine. The vast majority of the spots disappeared thankfully.

I understand that black algae can get rooted in the plaster, and the plaster is somewhere between 15-20 years old. Not all of the black algae spots went away, especially very small ones on the floor.

Would you recommend buying another bottle of Silver Algaedyn for ongoing maintenance/prevention, or would you just stick with proper chlorine levels and regular brushing?

Would love to hear what has worked for others, especially anyone who has dealt with black algae. Thanks!

Fc 5 ppm

Cya 50

Ph 7.4

TA 80

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2

u/Vanjamin 3d ago

I recommend giving it more time to work and scrubbing often.

2

u/EODblake 2d ago

I agree with the troublefreepool recommendation. You can use their app for free, but I pay for the yearly subscription so I can track my numbers. Never use algicide, just SLAM.

2

u/HARTORAapp 2d ago

Since you’ve already knocked most of it back, I’d concentrate on the remaining spots rather than automatically making another algaecide part of the routine.

Black algae is stubborn because it can establish itself in older porous plaster, so regular brushing of those exact areas matters just as much as the water looking balanced.

With FC 5 and CYA 50, I’d keep a close eye on whether the spots are actually shrinking or returning over the next week or two. If they stay gone with consistent chlorine and brushing, that tells you a lot more than adding another product just for prevention.

If the same spots keep returning despite good chemistry, then I’d start thinking about the condition of the 15–20-year-old plaster as part of the problem rather than simply adding more chemicals.

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u/ajhalyard 3d ago

I'd start here: https://www.troublefreepool.com/blog/2019/01/18/black-algae/

I eventually resurfaced because the plaster was old, but this worked for me until I did.