Stubborn (algae?) Stains on Plaster Walls

cmwerner

Member
Jun 7, 2020
10
San Jose, CA
Pool Size
18000
Surface
Plaster
Chlorine
Liquid Chlorine
Hi all - hoping to for some experience and guidance on stains that I'm dealing with in my pool that I haven't been able to identify or clean. Been fighting algae this summer, finally decided to do a SLAM, as well as taking up to mustard algae shock levels afterward.
  • 1 year pool owner, trying to learn as much as I can from TFP; pool is prob 30+ yrs old but recently (past 2-3 yrs) re-plastered
  • Algae problem seems largely solved, but I have these stubborn green/yellow stains that I can't get rid of
  • Been keeping the pool above the "Mustard Algae Min"; from the start of SLAM, pool has been at elevated chlorine levels for almost 2 weeks now
  • Bought a stainless steel brush which doesn't seem to help in removing the stains - though puffs of white debris when I brush - am I removing my plaster??
  • Rubbing chlorine tablet on stains - nothing
  • Rubbing vitamin C on stains - nothing
  • Stains are primarily along the South (North-facing) wall of the pool

Attached (poor) photo, really had trouble capturing this. It looks worse in person and spreads along the entire side of the pool in random patches.

Pool Math Log: PoolMath Logs (yes, I recognize the low FC levels earlier in the summer are the source of my issues in the first place - this was shortly after deciding to splurge for a TFTest Kit and learning how to use it and the app). For context I was previously using Trichlor tabs but have switched over to liquid chlorine given my CYA levels getting a bit higher than they probably should be.

Thoughts?
 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
44,831
Laughlin, NV
Pool Size
6000
Surface
Fiberglass
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
Any chance it is copper? Algae does not stain. A SLAM will remove it.
Any products used in the past with Blue in the name? Algaecides? A heater? An Ionizer?
 

cmwerner

Member
Jun 7, 2020
10
San Jose, CA
Pool Size
18000
Surface
Plaster
Chlorine
Liquid Chlorine
Any chance it is copper? Algae does not stain. A SLAM will remove it.
Any products used in the past with Blue in the name? Algaecides? A heater? An Ionizer?
I don't think so... I haven't used Algaecides (thanks TFP for steering me well away) and don't have a heater or ionizer. Shouldn't be any copper plumbing anywhere.

FWIW (I know, not worth much!), before I purchased my test kit and had Leslie's (I know, I know... now) test my water, they reported 0 measurable Copper.
 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
44,831
Laughlin, NV
Pool Size
6000
Surface
Fiberglass
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
The Server Host is having issues with attachments. So wait a while and try again.

If you are absolutely sure no copper, then I am not sure. Algae does not stain.
 

cmwerner

Member
Jun 7, 2020
10
San Jose, CA
Pool Size
18000
Surface
Plaster
Chlorine
Liquid Chlorine
Hypothesis - though can't wrap my ahead around it... we've got pretty hard water here in San Jose (glass shower doors tell the story if you forget to wipe them down after even just 1 shower). My Calcium readings seem to be within the acceptable range, but certainly at the high end. Is it possible there is scaling on the walls, that formed in place around the same time as the algae was blooming? And now the algae is "trapped" in the scale? This wall (amongst others) is definitely where algae was regularly forming and I was brushing it away before finally committing to the SLAM.

Would that explain the "white clouds" I get when brushing with the stainless steel brush? Or am I literally removing plaster when I see that (ie, time to stop brushing so aggressively)?
 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
44,831
Laughlin, NV
Pool Size
6000
Surface
Fiberglass
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
If your water chemistry is not in the scaling tendency area of CSI, it is very unlikely to have scale under water. You might get it at the water line or just above, but not underwater. If there is scale on the walls, it will feel like sandpaper. It does not form in a smooth fashion.

The clouds you are seeing are your plaster --------
 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
44,831
Laughlin, NV
Pool Size
6000
Surface
Fiberglass
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
Are you sure that area is just not a plaster defect or an area that cal hypo or other chlorine was dumped and not well mixed in? Does not seem to be any color to it at all.
 

cmwerner

Member
Jun 7, 2020
10
San Jose, CA
Pool Size
18000
Surface
Plaster
Chlorine
Liquid Chlorine
Yea so that’s the thing - it’s possible it’s been there all along and I’m now just hyper sensitive about it after the algae bloom.

That said, and it’s really hard to capture in the photos - it definitely has a green tinge to it across that entire south wall of the pool. Including another photo. Again, not super easy to discern but it’s there! (The green-blue transition is sort of diagonal across the middle of the photo; the “stain” goes all the way up to meet the tile and skimmer opening)
 

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