Black mystery stains


LifeTime Supporter
I have 2 large and several small ones. The large ones are quarter sized. I must have some small chips in the gelcoat where it starts because it's always the same places. They flare out with water flow. I can spot treat with citric acid and they fade or go away but come back in a month.
Water is crystal clear. Any ideas?
ch-300 (was at 150)
cya 40



LifeTime Supporter
Bama Rambler said:
Those look like stains from exposed metal. Especially given the fact that they always come back in the same places.
Don't know what metal would be in my fiberglass. I have the 2 larger areas and 10-15 very small ones. I figured it was some type of reaction with chems and pin holes through the gel coat. Just not sure what. At least I can get rid of them. I'm going to try a plastic bag on a pole with a very small hole filled with citric acid and hold the hole over the areas. Should work and use much less acid to rid the stain.
I'm still listening.
I have the same black dots with the flare out "tails" that you do. I have several but they are only as big as the smaller one in your bottom picture. Have never figured out what they are. I have tried to find pictures of what cobalt staining looks like but haven't been successful. They are on the bottom of the pool and the sides. I did try a vitamin c tablet on some on the ones on the sides, they fade for only a moment then reappear. I will be following this thread to see if you come up with anything.


TFP Expert
Mar 2, 2011
Most likely to be cobalt staining. Cobalt is used as one of the chemicals in some fiberglass resin. A reducing agent, such as ascorbic acid or oxalic acid might help temporarily, but the stains are likely to reappear. Using a higher calcium level and a sequestrant might help reduce reoccurrences. However, I think that the pool would need to be refinished with a high quality gelcoat to prevent future reoccurrences.

Of all the raw materials used to manufacture fiberglass pools, gel coat is by far the most expensive per pound. If a manufacturer wants to cut costs, this is a good place to start, because on a brand new pool, ordinary gel coat looks just exactly like the best gel coat that money can buy. But just wait 24 or 36 months and you will see a big difference. Premium gel coat won’t produce water blisters and black cobalt stains, and ordinary gel coat will.