Black / Dark Areas - Deep End of Salt Water Gunite Pool

[bbdude]

Hi All - Happy Friday! See attached photos. I noticed this recently in my deep end ... doesn't seem to be anywhere beyond this one area. Honestly can't tell you how long it's been there, but pool is a year old. Doesn't go away with brushing. My test from this morning is below and this is approx what my chemicals have been all year.

I don't think it's algae, but any idea what it might be? Closing pool within the next few weeks so want to address it now (if I need to). Thanks in advance!

FC - 9
TA - 70
CYA - 80
pH - 7.8
CH - 400
Salt - 3800
CSI - (0.09)

 

[Texas Splash]

I would first try the two easiest at-home DIY tests:
- Sprinkle some cal-hypo to see if it's organic; or rub a chlorine tablet on the area, but do not leave it in one spot.
- Rub a Vitamin C tablet on it to see if it's iron

 

[bbdude]

I would first try the two easiest at-home DIY tests:
- Sprinkle some cal-hypo to see if it's organic; or rub a chlorine tablet on the area, but leave it in one spot.
- Rub a Vitamin C tablet on it to see if it's iron

Thank you, @Texas Splash ! I have some chlorine tablets so going to rub one on it ... what do you mean by "leave it on one spot"? Do you mean leave the puck on the area I rubbed?

 

[Texas Splash]

what do you mean by "leave it on one spot"? Do you mean leave the puck on the area I rubbed?

Rats! I meant do NOT leave it in one spot. I'll edit that post. Sorry

 

[bbdude]

Rats! I meant do NOT leave it in one spot. I'll edit that post. Sorry

Should the results of rubbing the tablet against it be immediate? If so, assume it's organic? If it doesn't change, then I'd proceed with Vitamin C to determine if Iron?

 

[Rancho Cost-a-Lotta]

If you have a Leslie's nearby, have them test for copper/iron. TFP normally doesn't normally promote pool store testing, but there no other low cost options for metal testing.

 

[Texas Splash]

Should the results of rubbing the tablet against it be immediate? If so, assume it's organic? If it doesn't change, then I'd proceed with Vitamin C to determine if Iron?

Results can vary depending on any other factors (i.e. scale), but shouldn't take too long. You can do either test at any time.

 

[bbdude]

Tested the water at Leslie's and no iron concerns. They said my CH was 147 ppm and it should be between 200-400ppm. Not sure how it translates from my TF-Pro Salt that says 400 (which is ideal range per pool math).

Will try the tablet and Vit C this weekend ... thank you both!

 

[Texas Splash]

They said my CH was 147 ppm and it should be between 200-400ppm. Not sure how it translates from my TF-Pro Salt that says 400

That's another example how bad pool store testing can be, We'll watch for your tests. Enjoy the weekend.

 

[Newdude]

They said my CH was 147 ppm and it should be between 200-400ppm. Not sure how it translates from my TF-Pro Salt that says 400

You went there on a specific mission for metals. Anything else they said was to go in one ear and out the other. Always try not to spit up a little and just smile and thank them.

Heck, even the metals test is sus, but we have no other free options, or even reasonably priced ones.

 

[reggiehammond]

Tested the water at Leslie's and no iron concerns.

What was result for copper?

 

[bbdude]

Hey All - Thanks for the input and support.

Leslie’s test showed no copper / no iron.

I couldn’t scrap the black stuff with a finger nail, but chlorine puck did (very slowly) cause it to slowly go away. It’s a pretty big spot … maybe 3’x2’ or so.

Any recommendations? Otherwise going to the bottom 100s of times is in my future haha