Identify Stains

[nolefan]

I have been developing these mystery stains all over the bottom of my pool. They do not brush off, they do not feel raised or rough. The only thing I can think of is that the ph was allowed to get high (for me anyway) and remained above 8.0 for a while (not sure how many days). The chemicals were checked weekly and brought back to normal. But I am realizing the ph climbs faster than weekly . The TA never got over 100 at any time and hovered around 90. I have been adding acid every few days to keep it at 7.4 to bring the TA back down . It is currently at 80 and being carefully monitored.

Other chemicals:
FC: 11 (I have been raising this with the SWG just in case this is organic, but it doesnt appear so)
PH: 7.4 - 7.6
TA: currently at 80
CYA: 60
CALCIUM: 300

 

[jblizzle]

Try crushing up come vitamin C tablets in a sock of something and set it on the stain for a few minutes. If the stains lighten, then they are metals and you will need to do an Ascorbic Acid treatment to remove them and then use a metal sequestrant to prevent them from coming back.

If the do not lighten, try putting a trichlor tablet on them for a minute or so and see if they lighten. If so, then the stains are organic and elevated chlorine should take care of it.

 

[onBalance]

I don't think it has anything to do with the pH being at 8.0.
Another possibility: It looks like something was poured in at the stop step, settled on plaster, and slowly flowed over the step and then down to the other steps.
If it was an acid type chemical, it could have slightly etched the surface, and then dirt has adhered to that area over time.
Or perhaps an algaecide was added and left a stain. Many possibilities.
If the vitamin C and trichlor tab does not work as Jason (jblizzle) suggests, then simply try sanding with 100 grit wet&dry sandpaper. That will make the plaster more smooth in the process.