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Why Swimming Pools Turn Green, Brown, or Black After Adding Chlorine Shock

Barack has expertise in pool chemistry and pool maintenance. He writes in-depth articles about how to maintain pools for optimal enjoyment.

Learn why your pool is that unpleasant color and how to fix it.

Learn why your pool is that unpleasant color and how to fix it.

Why Is My Pool Green?

There are a couple of reasons why swimming pool parts and water might change into different colors, including green, brown, black, purple, and yellow. One of the major reasons that all pool owners know is algae breakout and another that very few know is metal staining.

1. Algae Breakout

Algae is always caused by low free chlorine levels and it appears in green, black, or yellow in color on pool walls, on pool floors, in the water, and sometimes on pool equipment, depending on which algae you are dealing with.

However, if the pool water and parts discolor specifically when you add chlorine or shock it, and the chlorine level is just okay, you are not having an algae problem. It could instead be because of heavy metal presence in your water that is causing metal stains.

Algae Test

You can carry out an Overnight Chlorine Loss Test (OCLT). To do the test, add a normal amount of chlorine to your water and take the reading. This should be done in the evening/night when there is no sunlight that consumes free chlorine.

Take the chlorine reading the following morning. If little chlorine is used up (0–1 ppm), the OCLT result indicates it is not algae or any organism that could have used up a lot of chlorine (above 1 ppm). Thus, it's can only be metal staining as a result of oxidization by chlorine.

2. Heavy Metals

When heavy metals including copper, iron, silver, manganese, and magnesium are oxidized by chlorine inside your swimming pool, pool parts and water will turn to different metal stain colors and some of the colors are similar to those of pool algae (green and black), especially when just starting to grow. This may confuse inexperienced pool owners trying to figure out between algae breakout and metal staining.

Copper is always the most common metal you will find in your swimming pool water. This is because it occurs naturally in well and some municipal water and also finds its way into the swimming pool water through some chemicals we use, such as some algaecides.

Copper turns to a light green color when oxidized by chlorine inside your pool. Other metals you may find in your pool—especially when using well water—are silver that turns to black, iron that turns brown/rust, and manganese and magnesium that turn to purple when chlorine is added to pool water with compounds of these heavy metals.

Heavy Metal Test

To be sure if you are dealing with pool algae or metal stains in your pool, there are two options you can use. The easiest way is to test your pool fill-water for these metals using a test strip that supports heavy metal tests like the LaMotte ColorQ Pro 11 test kit, which takes readings of all pool chemicals and common heavy metals you can find in your swimming pool, including copper, iron, biguanide, and biguanide shock.

This article will explain why a pool turns green when chlorine is added and what you can do about it.

This article will explain why a pool turns green when chlorine is added and what you can do about it.

Causes of Heavy Metals in a Swimming Pool

Depending on which fill-water you are using in your pool, a couple of heavy metals may find their way into your pool. The most common metals in swimming pools are copper and iron. Other metals like silver, manganese, and magnesium may only get into your pool if you are located around areas where these metals are mined and you are using well water to supply your pool.

Copper in particular is the most problematic metal in a swimming pool, and it will turn to light green when chlorine is added. When any heavy metal is oxidized by chlorine, your swimming pool water, and parts will discolor as a result of metal stain. As a pool owner, you have to know how metals get into your pool and how you can control or clear them.

Here are the sources of most common metals like copper and iron, which have higher chances of getting into your swimming pool.

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1. Adding Pool Chemicals With Heavy Metal Components

Using pool products with heavy metals as one of the major components is one of the ways you can get them into your pool. For example, constant use of some algaecides with excess copper will accumulate a significant amount of copper solution in a swimming pool over time. This may lead to pool water turning green when copper solution reacts with chlorine shock.

As such, it's always advisable to know your swimming pool's chemical components in detail and avoid chemicals that will cause additional problems in your pool.

2. Using Fill Water From a Well

Wells are commonly known to be one of the greatest sources of metals in a swimming pool, especially copper and iron. Iron reacts with chlorine to form iron III chloride, which is reddish in color. This is why your pool may turn to brown or rusty color when chlorine is added.

3. Using Metal Pipes and Pool Parts

During the construction of a swimming pool, copper, iron, silver, or any metal may be used in pipes and heaters. For instance, the use of copper parts requires a lot of pool care. This is because when the pH gets too low, metal parts (especially copper) will corrode into the water, forming a copper solution. Copper then comes out of the solution when oxidized by chlorine, causing a green color in your pool water and parts.

What Can You Do About It?

If you can't avoid using fill water from the well, using pool chemicals made of copper as one of the components, or using copper parts in your pool, you must be ready to deal with metals to avoid messing up your pool water.

Dealing with metals means frequent testing for metals and performing necessary treatment for every metal, mainly copper and iron.

I have been using Lamotte ColorQ Pro 11 to carry out different tests for metals such as copper and iron. This digital test kit is accurate and time-saving, since you can test all pool chemicals and common metals in your pool water in just one kit.

A few tips and tricks will keep your pool open for business all year.

A few tips and tricks will keep your pool open for business all year.

Steps for Clearing Metal Stains in your Pool

To clear metal stains in your swimming pool, simply follow these steps. I have included links to the products that I use.

  1. Get your free chlorine to 0.0 ppm by not adding more chlorine. This might take a few hours or days depending on the level of FC in your water.
  2. Add in the Swim's Polyquat 60 Algaecide after getting the chlorine to 0.0 ppm to prevent algae from growing in your pool during the process that might take a few days. Polyquat 60 or 60 Plus is the best algaecide for swimming pools, because it introduces no copper in your water and is also very effective in preventing algae.
  3. Get your pH to 7.2 using pH minus.
  4. Put your filter on circulation and add 1 pound of ascorbic acid for every 10,000 gallons of water in your pool to get stains away within 24 hours.
  5. Get the pH back using pH Plus. Then raise the TA using an alkalinity increaser, since ascorbic acid will reduce both.
  6. Add chlorine to raise your FC to around 3 to 4 ppm.
Don't we all just want our pool water to be beautifully clear?

Don't we all just want our pool water to be beautifully clear?

How to Remove Heavy Metals and Control Metal Staining in Your Pool

Treating heavy metals in your pool may be the only option for you when you have no other clean fill-water free of metals to use in the pool. There are a couple of ways you can take care of your pool so that heavy metals such as copper and iron will not affect your swimming pool when oxidized by chlorine.

Once you have an accurate test kit for metals and a few other chemicals we are about to discuss, your pool will never turn color due to different heavy metals.

1. Filter Well Water Before Entering the Pool

They say prevention is better than cure. Filtering your fill water from the well will be the best option if you want to have less work and more swimming. This is because water sourced from the well and some municipal water sources contain significant amounts of heavy metals, mainly copper and iron.