I have a problem with my pebble finish - random dark spots on the bottom, which looks like pebble discoloration. I think I've had this for a while, but I believe it became worse recently with more darker spots appearing. I've attached a few photos if that helps.
Here is my TF-100 results:
PH 7.5 (diluted pool water with distilled water to get a more accurate reading as suggested on the forum)
I did overnight FC loss test and it showed zero FC loss. Also, I put a chlorine tablet on the stain for a few minutes and noticed no color change. So, I guess it is not algae or other organics. Also, did a vitamin C tablet test by rubbing the tablet against the stain - no color change either.
Not quite sure what it is, so any help is greatly appreciated.