Black algae - algaecide worth it?


Well-known member
Jan 30, 2018
We have 40 year old marble plaster pool. The plaster is rough in areas and black algae keeps invading. I scrub it back from areas until it’s gone and dead, but it keeps reappearing in new spots.

FC 5-10
CC 0
ph 7.8
CYA 50
TA 50-60
Calcium 300
Salt 3500

temperature around 30 C. No problem with algae over winter.

Anything obvious I should change?

is algaecide worth it?

I know I have to beat black algae back the hard way, I just want to set up so it stops recolonising...

Texas Splash

Mod Squad
TFP Expert
LifeTime Supporter
The link above really does a good job clarifying steps to help eradicate BA. Also important to note is that your CYA is slightly below the TFP recommended level of 70. Since your water is fairly warm, it could mean more work for the SWG. I also doubt the SWG will be able to maintain a slightly elevated FC level required for BA treatment. In your case, with a current CYA of 50, it should be somewhere between 8-20. If you increase the CYA to 70, then the FC could go as high as 28 or below. Of course the brushing would still be required.

As for an algaecide, it won't help. Algaecides do not remove algae, and are only an insurance to help prevent the onset of algae - usually before winter. In addition, the elevated FC level required to kill BA would quickly consume any algaecide, so I wouldn't waste money on it. I also suspect that with a plaster surface of 40 years or so, it is very porous in some areas which makes the task that much more difficult. Be careful brushing the plaster as to not increase the potential for it to crumble or break apart. Kind of a dual-edged sword there I know.