Black Spots In Fibreglass Pool

Black Spots in My Fibreglass Swimming Pool - How To Remove Them?

Have you ever wondered why you see black spots in your fibreglass pool? These marks look highly sinister and suggest that something is drastically wrong with your pool. However, that isn’t always the case.

Black marks in your pool could be caused by several reasons – especially if they have a fibreglass lining. Luckily, the fixes to the issue are often as simple as the problem itself. So just how do you remove black spots from a fibreglass swimming pool?

Need Pool Chemicals?
Use the coupon code FEB15 to get 15% off pool chemicals on eBay!

What causes black spots to appear in fibreglass pools?

In most cases, pool owners believe that black marks appearing in their pool are a type of mould or algae. They are not wrong in this regard. After all, black algae can develop in pools made from all linings.

However, if you have a fibreglass pool, black spots could be an indicator that there are issues with the lining and that the fibreglass itself is deteriorating.

This happens thanks to osmosis. What this does is form small holes in the material lining where moisture seeps through each small gap. It’s something that is often not visible to the naked eye.

Diagnosing the problem

It’s easy to tell the difference between if you have an algae problem or an issue with your lining. You will need to look at the black spots and see what they feel and look like. If you have black algae in your pool, you will find:

  • The black spots are slimy to touch
  • They won’t react to chlorine
  • Mostly found in corners
  • Grows over time

This is a stark contrast to if you have issues with your fibreglass lining itself as the main things to look out for here are:

  • Jelly-like holes in the lining
  • Could occur anywhere in the pool
  • Doesn’t grow over time
  • Feels hard to touch

Thanks to the differences in the types of black spots, it shouldn’t be too hard to figure out which problem is affecting your pool. From there, you can proceed with a solution.

How to remove black spots from your fibreglass pool

No matter what option you choose to pursue, you need to think very carefully about what you use. Fibreglass can be reactive to several products and tools so always see if things are suitable before going at the problem hammer and tongs.

Removing Black Algae

If you have black algae growing in your pool, you will need to treat it with an algaecide. But, be very careful in what products you use. Some algaecides are highly acidic and may wear away at the lining itself.

It’s why it’s a good idea to check carefully what algaecide works with a fibreglass lining. You can use dedicated black spot algaecides – but follow the product instructions carefully.

Remember to switch off filters and chlorination devices before adding any algaecides. After leaving this for a minimum of 5 days, the algae should start to disappear and the pool will be safe to use again.

Repairing Bubbles in Lining

Black bubbles in the lining isn’t something that can be covered up. If you try painting over it, the spots will reappear in only a few months.

Therefore, the whole top lining in that area will most likely need to be replaced. What this involves is getting a technician in to examine the issue.

They will most likely remove the coating layers and strip the area back to the fibreglass itself. The issue will then be repaired and then re-layered and coated.

It is a much slower process than patching over it but it protects the pool's structure from long-term damage.

You can find more info in this article about non-algae black spots.

How to prevent black spots from developing

The way to prevent black spots from developing in your pool depends on the underlying cause of the problem.

Fixing black spots that are caused by issues with the fibreglass itself is a simple part of long-term pool maintenance. All you need to do is check the lining every few years and if you notice any issues, get it replaced.

Fibreglass pool linings should have a lifetime of at least 10 years so once you have the issue fixed, you shouldn’t have to worry about it for a long time.

If you are dealing with black algae, always keep your pool as clean as possible. This prevents the risk of algae building in the pool. Furthermore, check that the pool is well-chlorinated and maintains a steady pH level to ensure that the environment is hostile to algae!

Remember, in high enough levels, black algae can be dangerous to humans!

By following these simple tips, you will be able to remove black spots from your fibreglass pool quickly and easily. Once they’re gone, keep on top of your pool maintenance to prevent them from reoccurring!

Louis from Pool Advisor


A chemical engineer by trade, Louis is committed to debunking myths in the pool industry by explaining the underlying chemistry and making it accessible to all.