Plaster Stains - Not Iron or Algae?

[agriverbear]

I think this is best forum for this, please move it if not.

In February I noticed a several brown stain splotches in my deep end. I assumed it was organic and would go away. I now see about 3-4 more much smaller (1/4”) brown patches on steps. I tried the trichlor puck and vitamin C test and neither affected the stains. When I look at them up close underwater they look like iron/rust and will not scrape off. It doesn't seem like copper.

Thoughts on other tests? Next steps?

Pool is 20000 gallon plaster, 1.5 years old. I was on liquid chlorine up until early April when I switched to SWG.

I added Boric acid about 3 weeks ago.

Current chemistry

FC: 5
PH: 7.6
TA: 70
CH: 330
CYA: 60
Salt:3600
Borates: 50

Input water is exclusively rainwater

PH: 7.0
TA: ~10
CH: 0

Thanks

Mark

 

[Texas Splash]

Any chance you could post some pics?

 

[rt574274]

Had something like that last year. Turned out to be mustard algae. Invested in Yellow Treat algicide. Followed direction to the letter. U will need to brush the poolsides, and test your chlorine levels before swimming. I ran my pump 24 hrs. and brushed daily. For me it took a couple days. Make sure you maintain proper chlorine levels

 

[agriverbear]

It is hard to get a good pic, this is one of the small ones up on a step. I don't know much about mustard algae, but I noticed the first stain in February, water temps avearaged in the 50's. These seems very localized, rest of the water is clear.

 

[Texas Splash]

Hummmm. Only 1.5 years old. Have you reached out to your builder by chance? Since tabs and Vitamin C did nothing, you might try rubbing some dry acid on it if you have some - in a thin sock or nylon.