I guess those weren't rust stains. Now I've got nice black copper stains (I think)...

[aasarava]

Hey folks, what do you make of this: I've got a couple dozen brown-ish red stains, roughly an inch-long, in the deep end of the pool. The majority of it is in a cluster, not near any return. (See photo.) There are also a handful of similar stains scattered here and there, a few feet from the cluster.



They showed up a few weeks after I had some work done on the pool, including replacing the light that is in the deep end. So I assumed some tiny metal shavings from the niche may have caused the stains.

Based on this, and the rust color, I assumed they were rust stains. So... today I put a couple small socks of crushed vitamin C tablets on the worst of the stains... And now I have a mess. (See the second photo.) The stains have turned darker, while the plaster around them looks much cleaner -- making for a very visible contrast.

Imgur: The magic of the Internet
(Sorry, apparently I hit my photo quota, hence the link.)

First, any theories on what caused the stains in the first place?

Next, what do you all recommend for tackling this so it doesn't look so terrible?

Thanks!

 

[JoyfulNoise]

What has been added to this pool? Does your fill water have metals in it? City water or well water?

It’s somewhat unusual to go from no-stains to dark copper stains. You might might consider using Jack’s Magic Stain ID kit to see if you can better differentiate what’s going here. In the absence of having added any copper-containing pool chemicals, copper contamination is very atypical as water from a municipal supplier doesn’t typically contain any copper (iron is another issue). You might also take a water sample to the pool store and ask them to test it for metals (just so you don’t have to buy an expensive metal test kit).

 

[aasarava]

It's on municipal water. Unfortunately, I don't know exactly what's been added to it. We had a pool guy who could never fully get rid of the mustard algae, but I don't know if he ever tried copper-based algaecides. He did charge for phosphate remover a couple times a year when he cleaned the filter.

(I took over pool maintenance a month ago and the algae is all gone now just by keeping FC constant.)

In any case, would any of that result in the very obvious inch-long stains as shown in my first photo above? Seems like those causes result in a large, more diffuse stain? Or am I wrong about that?

Not sure if this helps in the identification, but the stains that had turned black after the vitamin C are now back to brown.

Will try the Jack's Magic stain kit and asking a pool store for a metal test. Thanks for the tips!

 

[JoyfulNoise]

It’s not uncommon to find pool guys using copper algaecides because they are cheap and will often get rid of the problem quick making them look good but leaving you with a long term mess. That would definitely result in diffuse tan/brown stains. As for those long narrow spots, it’s really hard to say.

The water test and ID kit will hopefully shed some light on the problem. If you do have copper in the water, your only effective fix is to dissolve the metal back into solution and then drain the pool as much as you can. You typically want copper metal ion concentrations below 0.3ppm or as close to zero as you can get.

 

[JJoe]

I had a couple of dark stains, that did not respond to vitamin c tabs. They were a little bigger than yours but the color looked similar. I was told that it was from rebar or a tie wire that was to close to the plaster, and the stain was bleeding through from the concrete.
Might explain why the stains got darker when you treated them? Exposed fresh darker stain after taking off faded outer layer?

Does not really explain why it just appeared, but I had some spots show up in the 1st 2 years of the pool, then 2 spots appear after year 3.

Fix was to chip off plaster, grind back the wire then replaster spot.