Fiberglass pool blackspot issue

[Ora]

Hi guys

Newbie here and recently moved into a new house with a Fiberglass pool. It was built in 2014. Mineral pool with chlorinators. Pump running 8 hrs/day.

As we recently moved in and found the poor seems a bit greenish and cloudy due to the recent heavy rain. After we thoroughly cleaned the pool, balanced the water and backwash the filter. We start noticing some black spots on corners and steps.

Added copper-based algaecide and shock the pool to raise chlorine to 10+PPM. The black spots keep spawning in multiple areas mainly at corners and steps. Always on floor, haven't found any on wall.

The black spot has a rounded head and can not be brushed out. Use my fingernail, barely scrape part of it. Now I am confused about whether it is black algae or osmosis stain.

Any suggestions about how to remove them are appreciated. Thanks in advanced.

Water test report: 8.Oct.2022
Total copper:0.5

 

[Texas Splash]

Welcome to TFP! Based on the use of copper-based products and a mineral system, you most likely are seeing the ill effects of copper staining. Copper produces some of the most difficult stains to treat, and the only way to remove copper from water is to exchange it, or exchange enough of it to not be a factor.

Be sure to bookmark our Pool Care Basics page and see the links below for more info. Let us knof if you have more questions. Enjoy the forum.

Stains in Your Pool

The stained pool is beyond embarrassing. Our in-depth Guide to remove stains in your pool will help you to remove the stains and prevent them in the future.

www.troublefreepool.com

Copper in Pool Water - Further Reading

www.troublefreepool.com

 

[Ora]

Hi Texas,

Thanks for the suggestion. I have attach photo of the copper based algaecide I used perviously. I pour half litre of it to the pool

When the blackspot start appears, there is 0 copper in pool. The recent increase of copper in water test happens after I pour the algaecide.

Is there anyway that I can identify the black spot is a copper stain?

Thank you

 

[Texas Splash]

Is there anyway that I can identify the black spot is a copper stain?

If you have some dry acid (i.e. pH Down), place some of that in a thin sock and rub the spot. Sometimes copper stains react favorably to dry acid. If that doesn't work, you can try to rub a vitamin C tablet on a spot just to rule-out iron.

 

[Ora]

If you have some dry acid (i.e. pH Down), place some of that in a thin sock and rub the spot. Sometimes copper stains react favorably to dry acid. If that doesn't work, you can try to rub a vitamin C tablet on a spot just to rule-out iron.

Thank you so much.

Is above method might damage the fiberglass surface? I am in panic mod atm as one of the pic shows a bit yellowish stain on surface. It is due to using tri-chlo tablet on top of the blackspot, as I originally think it is black algae which cause a bleach stain.

 

[Ora]

BTW, I tried use tri-clor tablet on top of the blackspot overnight which does 0 effect. That makes me think it is not black algae. And it stains my surface a bit

 

[Texas Splash]

I don't see the area where you left the tablet, but we never leave a chlorine tablet sitting on any pool surface. They are very strong (acidic) and can bleach any surface in just a few minutes. I doubt the staining was Black Algae. I'm leaning towards copper or perhaps some cobalt stain. When you try the dry acid method I noted above by putting some in a sock, rub it around in the area. If it's going to work you should see effects relatively quickly. While I doubt it's an iron stain, you can rub a vitamin C table directly on the spot. It won't hurt it at all.

 

[Ora]

I don't see the area where you left the tablet, but we never leave a chlorine tablet sitting on any pool surface. They are very strong (acidic) and can bleach any surface in just a few minutes. I doubt the staining was Black Algae. I'm leaning towards copper or perhaps some cobalt stain. When you try the dry acid method I noted above by putting some in a sock, rub it around in the area. If it's going to work you should see effects relatively quickly. While I doubt it's an iron stain, you can rub a vitamin C table directly on the spot. It won't hurt it at all.

Thank you so much mate!! I will grab some dry acid and Vc tomorrow and give a shot. Will update photo in this thread

 

[Ora]

Hi mate, some quick update here:

Try to use Vitamin C tablet to rub the black spot today. Some small black spot can be easily removed. Some large and old black spot require extra elbow grease, but in the end, it is been removed as well. Guess I can conclude it is a metal stain for sure.

Heavy rain recently, not able to test with dry acid. Will find a suitable time to test it as well.

My plan is:

1. Use stain remover to lift the stain and black spot from surface.
2. Add metal sequestrant to prevent metal from stain the surface again.
3. Get CuLator to remove metal from water.

Question:

1. What is the best solution(active ingredient) to remove metal stain from pool surface (Fiberglass)?
2. A lot of black spot remover for fiberglass pool use sodium metabisulphite as active ingredient. Can I use them ? or should I confirm the metal stain type first?

Thank you so much

E.G: