Is this mustard algae??

[Jjsmompa]

Help - last year - we had two small spots near our pool drain we thought might be yellow algae - with a lot of scrubbing they went away. Fast forward to this year and there is a large brown spot around the drain when we open the pool - the pool guys clearly didn’t do too thorough a job as I had to point it out to them. After some hard scrubbing it mostly went away. Now 2 weeks later - the pool has had the solar cover on and the wind makes A mess of it and I notice even more brown spots at the bottom and around the drain. No amount of scrubbing by me even with a wire brush removes it. So I call the pool company and explain that whatever it was is back and send a pic - I get a message back that the owner says it looks like its my plaster degrading and that my pool was built in 1993 so probably needs replastered. Except what they didn’t know is the pool was replastered by the prior owners in 2012. I have a hard time believing that my pool has some kind of brown stuff that was able to be scrubbed off by the pool guy but now the brown stuff is my plaster degrading......From what I’ve read about mustard algae is you should be able to scrub it off (Though it comes back easily). The other suggestion was that its a stain of some sort (from what I have not idea). Any ideas are very much appreciated!! Still plenty of FC in the pool form when it was opened and ph, Alk and CH normal. CYA is high at 100.

 

[mknauss]

Please post a full set of test results from your own test kit.

 

[Jjsmompa]

FC 9
CC 0
Ph 7.4
Alk 120
CH 250
CYA 120 (estimated since the marked lines only go to 100)

 

[mknauss]

Follow step 8 in CYA - Cyanuric Acid Test - Trouble Free Pool to test CYA above 100 ppm.

 

[Jjsmompa]

Shame on me for not thinking to do that. I am getting 120 with that method.

(Dumb question- does the CYA ever dissipate on its own if you stop using trichlor?)

 

[mknauss]

CYA does dissipate some. In 90+F water it can go down by 15 ppm or so a month. Cooler water tends to be more like 5 ppm a month. Many winterized pools lose their CYA while closed. Yours didn't.

To do any thing about your issue, you need to lower your CYA down to 50 ppm or less. Drain / refill is what you will need to do.

 

[Jjsmompa]

Thanks - so it’s more likely algae?

 

[mknauss]

If it brushes off it is. Stains do not brush off.

Does it poof away or does it take hard brushing?

 

[Jjsmompa]

The pool guy was able to get it off when he opened the pool but I cannot get any off even using a wire brush -

 

[mknauss]

OK -- read Stains in Your Pool - Trouble Free Pool