Small pin head size brownish spots

[stevest61]

Pool is 20 years old in ground fiberglass with salt and a sand filter. Pool is 11500 gallons levels are
FC 10
TC 10
ALk 120
PH 7.8
TH 250
CYA 50
Salt 3000
Water is very clear and sparkly
Had tested for metals at pool store and none present according to them
Tried ascorbic acid on spots. Rubbed and held for 10 minutes and no difference
Tried Mr clean pads. Bar keepers friend. Rubbed and no difference
Used bioguard stain magnet plus and nothing except phosphate infestation
We used orenda PR-10000 for phosphates
Took care of phosphates but still have spots
Pictures are the spots. Most the size of a straight pin head. You may have to enlarge pics to see spots. Spots all over except sides
Never had this issue until this year
Please help. Thanks to all

 

[HermanTX]

several members have a FG pool - maybe they can offer guidance @Texas Splash

 

[mknauss]

Floor or wall, or both?

Some look like they have a streak next to them. Possible fertilizer pellets?

 

[Texas Splash]

I did see an increase in those same type of spots in our shell this season. The only thing in my case was it might be related to some granular pesticide I laid down around the perimeter of the pool, but Vitamin C did nothing. I'm still in the experimental phase of treatment.

 

[mknauss]

Lots of threads to look at --- Search results

 

[stevest61]

Floor or wall, or both?

Some look like they have a streak next to them. Possible fertilizer pellets?

Floor and steps only no walls. No fertilizer around

 

[stevest61]

Lots of threads to look at --- Search results

Thanks I’ll take a look

 

[mknauss]

Floor and steps only no walls. No fertilizer around

I suspect it is something that fell into the pool water. From a neighbor, etc.

 

[stevest61]

I did see an increase in those same type of spots in our shell this season. The only thing in my case was it might be related to some granular pesticide I laid down around the perimeter of the pool, but Vitamin C did nothing. I'm still in the experimental phase of treatment.

Please let me know and I will do the same
We are trying different things also