Round Dark Spots

[J04s18W1771s]

I and several pool guys have been stumped for years. Hoping some crowd sourcing can dredge up new ideas.

Five year old in-ground pool. Started with a Frog and mineral system. Terrible results. Converted over to TFP recommendations over a year ago. Water is crystal clear and well balanced. Chem stains remain in the bottom. That’s not the issue. They will get dealt with once the true reason behind the post is resolved.

The issue is there are 3”ish round dark brown/black stains mostly on the walls. Have tried all the typical treatments. It’s not a typical (black, green, yellow) algae or chemical stain. I can remove them sometimes temporarily and sometimes permanently by “burning” them off with acid.

Included are a few photos.

Any thoughts?

 

[Texas Splash]

Based on the pool's history, my immediate and first concern would be copper. Have you tried applying some dry acid ( in a thin sock or nylon) to the stains? If the dry acid works, you know it's copper.

For everyone's benefit who may wish to reply, I would also include a full set of test results from your TF-100 (or Taylor K-2006C).

 

[JoyfulNoise]

Does vitamin C applied with a sock or nylon lighten them?

Isolated round stains in plaster are often caused by rebar in the gunite shell being exposed to the plaster. Over time, iron leaches through the plaster and it develops a rusty-brown dark color. Vitamin C tablets will lighten the stain but it eventually comes back.

 

[JamesW]

Maybe copper or iron. What was the original color of the plaster?

If it's rebar under the plaster, a strong magnet should stick to the spots.

Copper usually creates even turquoise staining that can be difficult to see because it blends in to the natural color of the water and some plaster colors.

Dark spots or stains usually happen when chlorine makes contact with the plaster and further oxidizes the existing copper stains.

If you can safely drain and refill, I would consider doing a sulfamic acid treatment and then draining and refilling.

Note that draining a pool can result in the pool popping out of the ground under conditions where the ground water is high.

 

[J04s18W1771s]

Thanks for your replies. Just catching up. This has been an on-going issue for probably the last 3 yrs. It has never cleared.

Test results from this morning are FC 6.5, CC 0, pH 8, TA 80, CH 250, CYA 90. Pool tends to run pretty consistent.

Texas Splash - It’s not copper. Though there is general copper staining from the Frog chemicals.
Joyful Noise - Tried vitaminC. Unfortunately it did nothing for these spots.
JamesW - The plaster is white. It appears blue in the photos due to the dark clouds. Photos in the sun don’t allow the spots to show.

Yes, tried Jacks Magic, inconclusive results. It’s baffling. It doesn’t appear to be leaching through from the gunite. I can’t rule out there was not an issue with the plaster mix.

 

[JamesW]

It's probably copper.