When you see little black spots on your fiberglass swimming pool, it could be a few things.  First, try to remove them with a vitamin C tablet.  If that doesn’t work, then have your water chemistry tested to see if you have a high phosphate level.  If you do, use a phosphate remover then balance the pool water to see if that solves the problem.

Cobalt can also cause spots like these.  The gel coat delaminates a bit from the surface and these spots can develop.  Just place a CuLator Ultra 4.0 into the pump basket to see if that lowers the metal level to remove the staining.

Gel Coat delamination does happen with some fiberglass pools.  Check with your pool manufacturer to see what they can do, but doing a phosphate, or stain treatment is much easier to deal with.

If you have any questions, give Periodic Products a call at 954-764-7654 to discuss.

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  1. I have black spots on the side rules and the bottom of my pool it Almost looks like holes coming through from the outside they just won’t go away if I take a scrubber and scrub them usually remains a brown spot It’s a fiberglass pool

    • Hello Dottie,

      Have you tried ascorbic or citric acid on the spots to see if they lift? If they lift then they are a metal stain. However, on some fiberglass pools, if the phosphates go up too high, there is an algae which forms in the delaminated areas (usually little circles) and can look black. If you lower the phosphates, that can help them go away. However, if the acids remove them, then try a Metal Eliminator Stain Kit to remove the metal stains, sequester the metals and eliminate the metals from the fiberglass pool water with CuLator. I hope this helps! Call us if you have more questions at 954-764-7654.

  2. My fiberglass pool water is crystal clear. Black spots are developing on the sidewalk. Any suggestions?

    • Mary,
      Thank you for the message.
      Sometimes those black spots come from algae. Do they brush off? Check your phosphate level, if it is too high, you may develop little black dots which go away when the phosphate level drops. Did you test for metals in the water? If you have metals, just place a CuLator 1.0 or a CuLator Ultra 4.0 into your skimmer or pump basket to eliminate the metals.

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