Unknown pool surface issue

TXaquaholics

Bronze Supporter
Feb 12, 2019
38
Austin, TX
I have a pool surface issue occurring that is slowly getting worse. New replaster a year ago (see signature). The pictures show the details but it's light brown/green areas at various locations. At the places of discoloration, the finish is a bit rougher than normal (the finish is acid polished). The locations where this is occuring tend to get little direct sun. All on the sidewall, nothing on the floor. I'm a retired engineer and a fanatic about my chemistry, almost always within +/- 0.3 CSI (a few outside that range during travel) and routinely well less than that (see online logs). Chlorine and acid are added daily manually. The finish is brushed usually once a week. The robot also does the walls every week or so. For these trouble areas I have tried hard brushing with stiff nylon bristles and also a SS brush. Nothing happens. A pumice stone barely phases it. Intense scrubbing with a trichlor tablet takes a little bit away. I really don't ever shock the pool as we have no algae issues. This has me stumped. Appreciate any thoughts or suggestions.
 

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JamesW

TFP Expert
Mar 2, 2011
20,712
Looks like typical iron with possibly some copper in the last picture.

Try vitamin C on a spot.

When is the last time you cleaned the filter?

Check for iron and copper stains on the grids the next time you take it apart and clean it.
 

TXaquaholics

Bronze Supporter
Feb 12, 2019
38
Austin, TX
I don't see a SWG in your signature and it says Chlorine. For a chlorine pool with CYA 60 your FC target is 7-9 with a minimum of 5. Your PoolMath log shows the FC below the minimum consistently.

I suggest you do an Overnight Chlorine Loss Test as everything points to algae.
Hi Allen,

Correct. No SWG. The 60 CYA appears to been an anomalous reading on my part. If you look closely at the logs, I've been running 50 since December and 40 since end of April. No reason for it to have made a jump from 40 to 60 but I left it in there and checked it again the following week, back to 40 (user error). The calcs show FC 3 as the minimum for that CYA. I have a detailed XLS I maintain and I always stay just above the minimum FC as recommended by TFP. I have min FC = 0.075 * CYA. My weekly measurements are typically on Saturday morning before adding chems, so FC will be towards the low end of the range at that point. For the past month, with sunny hot TX weather, the pool/spa has been consuming 36 oz of 10% chlorine daily. I'm thinking this is about right for the UV loading we have. I'll do the overnight test to confirm.

I agree with the possibility of algae, but the inability to easily remove it from the plaster surface confuses me. And the brushing hasn't kept it at bay either.
 
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TXaquaholics

Bronze Supporter
Feb 12, 2019
38
Austin, TX
Looks like typical iron with possibly some copper in the last picture.

Try vitamin C on a spot.

When is the last time you cleaned the filter?

Check for iron and copper stains on the grids the next time you take it apart and clean it.
James,

I did full tear down and cleaning of the filter May 4th. Nothing out of the ordinary was seen. I'll look into the vitamin C suggestion.

Regarding the colors you're seeing in that last picture, this is the stone pieces in the Wet Edge Tourmaline (see attached for sample). Never have had a metals issue in 12 years.

Guys, Just to be clear, the problem I'm referring to are those faint/very light brown blotches on broad areas of the plaster.
 

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TXaquaholics

Bronze Supporter
Feb 12, 2019
38
Austin, TX
It's probably iron. Did you try the vitamin C?
James,

Thanks for the reminder of the vitamin C test. I was going down the path that this was possibly a mild case of mustard algae since the stains are mostly on shady walls. But I'm always above the min FC for my CYA, the water is crystal clear, and most importantly, a trichlor tab did nothing for the stain. So, infact, the ascorbic acid did inteed quickly lighten the stain. So it appears this is an iron issue.

From 2008 thru 2018, I was a trichlor tab as well as Natural Chemistry PoolPerfect + PhosFree and Scale Free user. Pool water was never changed out and fill water is City of Austin. So I'm thinking that possibly the Scale Free kept the iron under control. Pool/Spa was resurfaced early 2019 at which time I switched over to TFP methodology. The only chemicals I've been adding since then are liquid chlorine, muriatic acid and baking soda. So now I need to do more reading and research to best understand how to remove the stains in the near term and keep the iron under control long term, again using TFP methods. I see Leslie's has all kinds of potions to address the issue but I'll hold off on that for now.