How to Determine the Cause of Brown Stains in Your Swimming Pool and How to Clean Them

Over this last weekend, I took some time to get some much needed exercise in a local gym’s lap pool. The gym is clean and well-maintained, but I couldn’t help but concentrate on the giant, brown stains all over the bottom of the pool. What causes these stains, and how do we clean and prevent them?

Brown swimming pool stains have many sources. Algae, metals, calcium buildup, and just plain dirt. While we touch on pool maintenance in this article to help prevent staining, we’ll spend some time talking about what to do if you do have stains.

Often times, pool owners determine the cleanliness of their pool just by “feel.” Does the water look clear, and not smell or test too bad? Then they think it’s fine – just add some chlorine now and then. But there’s more to it than just that. Pool chemistry is pretty complicated and requires the monitoring of several factors. If it’s not kept on top of, you can end up with stains like above in an otherwise clean pool.

In this instance, brown pool stains can have one of a few causes: algae, metal, or dirt buildup. But how do you determine which? The first way is by color. A greenish-brown usually indicates organic – either algae buildup or organic materials such as leaves were allowed to sit on the bottom for some time. If it’s closer to a brown-black, and is found on the sides of the pool as well, then you might be looking at metal. And brown stains concentrated on the bottom can likely be attributed to dirt stains, typically enhanced by calcium buildup.

The first step to deal with organic materials is to shock your pool, and scrub it. This will take care of organic material present in the pool, and the scrubbing will take care of the stains. If it’s been built up over time, it will require some elbow grease. Maintaining proper pool chemistry is important to prevent this issue in the future.

What Are Those Brown Stains in My Swimming Pool


If the stains are of a metal nature, you can quickly check by scrubbing a stain with a vitamin C tablet. If the stain fades or disappears, you know you have a metal issue – most often iron or copper buildup. You can treat this with absorbic acid, or any specialty chemicals suited for metal at your pool supply store – and a good scrubbing. Preventing this in the future, again, is down to maintaining proper water chemistry. Certain areas have high concentrations of metals in their waters, so you’ll have to stay on top of it.

In the case of the gym pool above, the problem is very likely called ‘dirt scale.’ This is caused by a layer of dirt being sealed over by a layer of calcium buildup. Basically, the calcium acts as a clear coat over the top of the dirt-stained pool surface. This is common in highly-trafficked swimming pools such as the gym pool. It’s more difficult to remove than other stains, but it still can be treated. You’ll have to lower your PH to be more acidic – 6.8 to 7.0 (don’t swim in the pool during this time!) and scrub regularly. This process will need to be repeated several times over the course of a few weeks. To prevent in the future, again, keep on top of water chemistry and keep that pool scrubbed.

Stains in your pool can be ugly and difficult to deal with, so staying on top of your chemistry to prevent them is important. Keeping your pool PH and chlorine levels monitored, as well as regular vacuuming and scrubbing can go a long way to ensure your pool is beautiful and clean for years to come!

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