Black spots on concrete pool

[moore887]

I've seen these spots many times. Very common on warmer pools, dirty filters, poor circulation, short pump run times...the reasons go on and on. It is what it is. Black spots with a squishy surface that will burst and turn green when you rub them with your finger. Unfortunately my fingers don't go three feet down into the pool and here in lies my problem. One of the pool gurus will call them diatoms and have a specific online course for treating this but I've already taken their advanced chemistry course and I don't think I'm up for throwing money at mickey mouse courses who's accreditations disappear after a few months only to be replaced by another unrecognized qualification. Bottom line is, I have about five pools which I see this regularly in and I need to compile a strong attack plan to be able to deal with the issue and be able to deal with my customers' concerns.

Any help from the usual suspects is always appreciated!

 

[kul]

Curious - I thought pump times don’t matter much? Is that an issue because all the pools you service are SWG?

 

[moore887]

It would be nice if all the pools I serviced had SWG systems! I think about 30% have them. Some have failed over the years and the owners are reluctant to replace the cells or units. Many pools we have to leave chlorine at the pool so that the customer can do an extra top up dose during the week. All my pools are running 50ppm borates. That's my safety net.

 

[Texas Splash]

"887", I think before we get too far ahead of ourselves you should post a full set of water test results. Your Poolmath logs haven't been updated for a while. Squishing black spots into a green substance sounds like algae, but let's see those test results first.

 

[moore887]

Fc.5
Tc.5
Ph 7.6
Alk110
Cya 40
Ch 250
Salt 1250
Temp 78
Borates 50
I have used calhypo in this 10000 gal infinity edge pool the last few weeks because he believes there is a green tint in the wall and it could probably do with the calcium. I just washed yhe pa120 filter. I also leave one gallon of chlorine at the pool for him to put in on day 3 to stop the pool from going to zero. 8 hours runtime on the single speed pump.

 

[Texas Splash]

In your results above, is that 0.5 of free chlorine?

 

[moore887]

Yes, That was last weeks results. I added 1 gal of chlorine and left one gal at the customer's house for him to put in 3 days later

 

[Texas Splash]

With an FC that low, it appears that pool has algae. Might even be Black algae. A SLAM Process is warranted.

SLAM - Shock Level and Maintain

We’ve all had to deal with a green pool. Clear algae from your pool with a SLAM! Read here to use shock level and maintain to clear your pool.

www.troublefreepool.com

Black Algae

This guide provides a step by step procedure for eliminating black algae in your concrete or gunite pool. Read full post here.

www.troublefreepool.com

 

[moore887]

Just a quick update. I went to the pool the day before yesterday. The owner said that he had taken a sample of water to the pool store where they tested for chlorine and ph. Results were 2.0FC and Ph 7.8 The pool store told him to put 6 cups of Muriatic acid in the deep end in front or a return and then put in 2.5 gallons of chlorine. I am not too sure if I would agree with that much Chlorine but I'm pretty sure that's going to kill off those black spots after I break them slightly with a steel brush.

 

[kul]

Maybe ask them if they go straight from ocean water to pool water? If they do, I think I read on here that that could result in black algae in the pool eventually. For future they should consider showering/changing swimwear. If I’m wrong about the origin or understanding of black algae please feel free to correct me.

 

[JamesW]

Probably black algae from lack of brushing and insufficient chlorine.

 

[Katodude]

I would bet on black algae. Its south Florida. That stuff is all over the place. I know if my FC dips thats what I will get first.

 

[JamesW]

I am not too sure if I would agree with that much Chlorine but I'm pretty sure that's going to kill off those black spots after I break them slightly with a steel brush.

Black algae is not that easy to kill.

You are also probably transferring it from pool to pool on your equipment.

You probably need to maintain the FC higher on all pools and brush well at least weekly.