Black Spots in Shallow End

[Ross939]

Hello everyone: I was wondering if these spots look familiar to anyone? I've got the pool looking pretty good so far, 10,000 gallon in ground plaster pool, keeping chlorine at about 4pm TA is at about 160 ppm, PH crept up to about 7.9 and added 1.5 cups of Acid to get it down. These spots are really tough to get off. I have to scub with a steel brush and it's getting worse. Any suggestions?
[attachment=0:1rujjzpn]ImageUploadedByTapatalk1339377525.308580.jpg[/attachment:1rujjzpn]

 

[mpacheco72]

Rub a chlorine puck agains them and see if they disappear.

 

[mpacheco72]

I had similar spots in mine and I had to use a butter knife to scrape them all off. I tried the stainless steel brush, but I didn't have the upper body strength to really get them off. Then I used a chlorine puck to get the remainder. If it's what I think it is, you will definitely have to reshock your pool until you pass the OCLT after you manually remove them all.

 

[Ross939]

Thank you, I'll try that. A friend who had similar spots recommended a pumas stone, the kind that are on a handle that will fit on to your pool pole. My concern would be the pumas might etch the plaster.

 

[257WbyMag]

Looks like black algae to me. What is your CYA level?

 

[Ross939]

CyA is 90. I have eliminated pucks and calcium based shock, and have been sporadically draining and replacing water. Water's expensive here so I can't flat out drain it. PH keeps wanting to climb more than usual, as well. Just added close to 2 cups of acid to get it down to 7.4 from 7.9.

 

[mpacheco72]

Did you try the removal technique??

 

[Ross939]

Yes, worked on it some last night. While I'm concerned getting my CYA levels higher with the puck, I did go over some spots with the puck, and let the puck rest on a couple spots. I still have more work to do today but it's looking better.

 

[mpacheco72]

Good, I'm glad to hear that it's working. Since the puck isn't being completely dissolved you're probably ok.

 

[257WbyMag]

If your CYA is 90, then as long as your FC is below 7, you have a pool environment that promotes the growth of algae. An FC of 4 really isn't doing anything with a CYA that high.

pool-school/chlorine_cya_chart_shock

Black algae is very difficult to rid from your pool. It takes high FC levels for extended periods of time and lots of aggressive brushing to the affected areas. You either need to lower your CYA by water replacement and start shocking at a more manageable level (best), or you are going to need to raise your FC to 35 (not reasonable really) and brush like crazy in the hopes that the BA goes away quietly. Please read the following to rid yourself of this problem.

pool-school/pool_water_chemistry
pool-school/shocking_your_pool
pool-school/defeating_algae