Black Pool Water

Pool Water Is Black - How Do I Fix It?

If you’ve noticed that your pool water is black, you may be startled or even confused. Black pool water is indeed a rare occurrence, but it is not something to be afraid of.

There are a few potential causes for this condition, but regardless of the one that is causing issues in your pool, you’ll find the solution right here.

In this article, we’ll cover the causes of black water, how to diagnose the cause of it in your pool, how to restore your pool’s water quality to crystal clear, and how you can prevent your pool from turning black again in the future.

Causes of Black Pool Water

The causes of black pool water can typically be narrowed down into two main categories: an accumulation of dissolved heavy metals in your pool, or an extreme overgrowth of algae.

Dissolved Heavy Metals

Certain dissolved heavy metals in your source water can interact with other pool chemicals and cause your water to appear black. These metals can also settle out on the surfaces of your pool and can become stains.

Metals are often found in various pool chemicals, so think back over the last few weeks and make a note of what chemicals you’ve added to your pool.

Testing kits for heavy metals can be hard to find, especially if you are looking to test for multiple types of metal. For this reason, we recommend that you make sure you are not dealing with an algae problem before you try to find remedies for heavy metal staining.

Algae Overgrowth

Algae overgrowths are the most common cause of black pool water. The colour itself can be due to the specific type of algae present, and can also be reflective of an organic debris buildup.

Black pool water can be caused by many strains of algae, but the most common two types that result in this colour are blue-green algae and black spot algae.

Many pool owners who are unfamiliar with black spot algae typically describe it as a black mould growing on the surfaces of their pool in small clusters. Blue-green algae is more traditional in appearance, and will slowly build up as a slime-like layer on the surfaces of your pool.

Once an algae infestation has developed on the surfaces of your pool, the amount of algae present in your water column will skyrocket. This high-algae content is what causes a noticeable change in the colour of your pool’s water.

Problems with pool upkeep, equipment, or chemistry are usually to blame in these cases. Before taking steps to further investigate an algae problem, ensure that your filter and pump system are operating properly.

Your filter should be run for an absolute minimum of six hours per day, but more time may be needed for pools in areas with greater levels of natural debris. If your pool’s pump is set on a timer, make sure that this unit is functioning as intended.

Organic debris is a large contributor to algae infestations, as it provides nutrients that the algae needs in order to multiply. Debris can also contribute to turning your water black. Most organic debris such as wood or leaves slowly break down in the water and release tannins, a natural and harmless substance capable of changing your water’s colour.

Is Black Pool Water Safe?

Because black pool water is caused by either a high concentration of heavy metals, or an extensive overgrowth of algae, this water is considered unsafe for swimming. It’s important to not allow any swimmers into your pool until this condition is resolved.

Avoid swimming in or coming into contact with black pool water at all costs. If you must come in contact with the water in order to perform maintenance, use protective eyewear and wash any skin exposed to the water thoroughly with soap and water.

How to Fix Black Pool Water

Determine Cause Of The Problem

The first thing you will want to do when attempting to fix your black pool water is to run a full chemistry test. Use a kit that tests for free chlorine, total pH and total alkalinity.

When diagnosing an algae issue, it is also useful to test your phosphates. Phosphates are introduced to your pool through debris and normal use, but when they accumulate in high amounts, they act as an abundant food source for algae.

You’ll also want to have a close look at the surface of your pool and see if the black stuff resembles algae, or whether it looks more like metal staining.

Check for signs of metal staining that can develop along the lining as green, black, brown, or even purple patches. If you notice any of these areas, attempt to scrub them lightly. If pieces come off, this stain is algae or grime. If it doesn’t budge, it’s a metal stain.

You can try stain remover pens on the black stuff and if it responds, that’s a sign that it’s metal staining. If it doesn’t respond, it’s most likely algae.

Treating Metal Staining

For treating metal stains, we recommend the Lo-Chlor Multi-Stain Remover.

Unlike other products that only remove iron or copper stains, this remover contains compounds that work on all four of the most common heavy metals found in pools. That way you won’t need to spend more time testing your water or purchasing more test kits.

Treating Algae Overgrowth

Shock Your Pool + Remove Phosphates

After diagnosing the problem, you will need to do a chlorine shock treatment. You can use this chlorine shock calculator to help you figure out an appropriate dose.

Note that extensive algae problems typically require multiple shock treatments. You may even decide to double or triple the dose of shock that you use in your pool, but be aware that this can have other effects on your pool’s chemistry, depending on the type of shock you use.

When treating an algae infestation, you will also need to thoroughly clean or even possibly replace your filter media.

Remove Phosphates

If you have high phosphates, for the next step you will want to use a phosphate removing product. This will speed up the algae removal process. Our favourite product for this is Starver M, which is concentrated and is easy to use.

Apply Algaecide

Stubborn growths that persist after these treatments and cannot be vacuumed off of surfaces easily may be a strain of micro-organism known as black spot algae.

This type of algae is particularly difficult to remove, and requires lots of manual scrubbing in addition to chemical treatments. We recommend an algaecide known as black spot algae killer, which is specifically designed to be tough on this strain.

How to Prevent Black Pool Water

Black pool water caused by algae can be prevented by keeping your pool adequately maintained. Be sure to vacuum your pool when debris has accumulated and empty your skimmer baskets regularly. You will also want to keep an eye on your pool’s chemistry, including those phosphate levels.

For black pool water caused by heavy metals, you may want to consider a stain inhibitor. These products bind to heavy metals in your water, preventing them from oxidising on the surfaces of your pool or from affecting your swimmers.


Although black pool water can be intimidating, once you have figured out the source of the problem it is easy to find the right solution.

Do you have any questions about black pool water or how to get rid of it? Do you have any personal experiences with black pool water that you’d like to share? We’d love to hear from you in the comment section!

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.