brown residue stuck to bottom and sides

[Timothy5950]

I have been treating with slam method for 4 days, keeping chlorine at 20 ppm. 16000 gallon pool with CYA of 45. Green algae is gone, water is clear but there is brown residue spots on sides and around lights. It is also accumulating around the edges where side meets the bottom. It does not brush off or vacuum and it is expanding.
Need help with this.

 

[mknauss]

Welcome to the forum.
Can you post or input to your linked Poolmath data your current water chemistry test data?

 

[Timothy5950]

Fc= 24.5
Ph 7.7
TA 150
CH 125
Cya 45
CSI
I don’t have a log calculator. Temp is 92 but we are having 100 to 107 every day for last few weeks

 

[mknauss]

OK -- you have Poolmath linked to your Membername but no data in it.

Does not look like you should get scale. Your CH is very low and likely causing your plaster to be damaged.

Next would be to test for iron staining. Can you post pictures of the spots?

 

[Timothy5950]

the CH from my last test at Leslies was 197, I think I had trouble determining the endpoint in my K-2006C test kit

 

[mknauss]

That looks like copper. Or dirty scale.
Are the areas rough? Same or different than the non stained areas? Be sure it is not gunite with the plaster worn away.

 

[sktn77a]

Test a small area with an ascorbic acid tablet. If the stain goes away, its probably iron. If it doesn't it may be copper. If iron, dump in 1/2lb - 1lb ascorbic acid , that should take care of it. If it's copper DON'T use the ascorbic acid. Did they test for iron or copper when you had the water tested at Leslies?