White spot on bottom of fiberglass pool

[ba67]

After we completed a SLAM, we discovered that there is a huge white spot in the deep end of our white fiberglass pool now. I included a picture below. I don’t know if I caused this by adding liquid chlorine too quickly during the SLAM or if the company that opened up our pool caused this by putting in powdered shock and not brushing well afterwards. (I didn’t realize that they added powdered shock at opening until I got the bill a few days later.) The bottom of the pool wouldn’t look as discolored if it weren’t for this huge white spot. Is there anything that I can add to the pool to bleach out the rest of it so this huge white spot doesn’t stick out like a sore thumb? I don’t want to add anything that could potentially harm my gelcoat since we already have gelcoat issues. I've brushed the discolored area around the spot numerous times and it hasn't helped.

 

[PoolStored]

If chlorine cleaned a portion of your pool, the other darker spots are likely organic staining.

To remove organic stains raise the FC up between 60% and 100% of shock level for your CYA, and brush often, daily if you can, or more. Should slowly lift the organic staining.

 

[ba67]

If chlorine cleaned a portion of your pool, the other darker spots are likely organic staining.

To remove organic stains raise the FC up between 60% and 100% of shock level for your CYA, and brush often, daily if you can, or more. Should slowly lift the organic staining.

Could it harm the gelcoat if I got FC up that high? Should I use powdered Cal-hypo shock so I can put the chlorine where I want it and then brush it in?

 

[PoolStored]

I wouldn't put cal-hypo on the surface, I wouldn't hesitate to raise FC to 75% of SLAM for a week and brush brush brush...might just lift those stains...

Let's call @Texas Splash, he's one of our "go to" FG guys, and see what he says...

 

[Texas Splash]

How old is this pool?

 

[ba67]

How old is this pool?

I think it is around 19 years old.

 

[Texas Splash]

So a few things come to mind. The gelcoat is seasoned and perhaps a bit more prone to chemical changes than when it was brand new. Perhaps a chemical settled in the deep end and was responsible, hard to know. What we don't know is if it bleached the original color or actually removed a discolored film due to organics, scale, or a combination of the two. I don't think you could just try to make everything lighter. It may not work out, and even if it does may not be uniform.

I would be inclined to pick a small area to test. A small spot that won't upset you if a test doesn't work out. Rub the area with a chlorine tablet or sprinkle a small amount of cal-hypo there to see if it lightens a bit. Even try rubbing an area with a Vitamin C tablet to rule out iron. At this point I think it's worth exploring all those options. It may give you a better understanding of what happened and how to go forward.