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

Updated on June 19, 2019
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Barack is an expert pool chemistry guy and experienced online-based pool maintenance assistant via in-depth articles that top search results

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. | Source

Why Is My Swimming Pool Discoloring When Shocked With Chlorine?

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.

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 algae problem. It could instead be because of heavy metal presence in your water that is causing metal stains.

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.

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 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, beguinide, and beginide shock.

Secondly, 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.

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 a 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.

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 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 construction of a swimming pool, copper, iron, silver, or any metal may be used in pipes and heaters. For instance, 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.

Steps for Clearing Metal Stains in your Pool

To clear metal stains in your swimming pool, simply follow these steps:

  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 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.

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. And in some cases, you may find some more heavy metals, such as zinc, magnesium, aluminum and silver in your pool water.

Each and every metal is oxidized and produces a solution of unique colour when chlorine is added to a pool with these metals:

  • Copper: Produces a light green solution or stain.
  • Iron: Produces brown/reddish/rusty coloured solution or stain.
  • Manganese and Magnesium: Produce purple solution or stain
  • Silver: Forms a black colour stain or solution.

Whichever metal is causing problems in your pool, you can use a pre-filter to filter out these metals before they enter your swimming pool, where they will most likely be oxidized by other pool chemicals and stain your pool or change its water colour.

Moreover, when you prevent metals from your pool, you will save lots of money from buying pool chemicals that are needed to take care of metals once they are inside the pool. Since I started my pool business, I have been using the Pleatco Disposable Pre-filter. I consider it the most effective pre-filter, because my clients who are using metal ridden fill-water have been sending me positive comments about saving on chemicals and time for maintaining their pools.

Pleatco's Pre-filter is also ideal since it can filter a one-time water capacity of 40,000 gallons and can easily thread onto any standard garden hose. This pre-filter can also be used for in-ground, above ground and spas sourcing from metal-laden water.

2. Using Chemicals to Treat Heavy Metals in a Pool

The final option and the most tiresome method of dealing with metals in a swimming pool water is using chemicals. Besides being tiresome, these chemicals may be a little expensive and might need extra care to avoid more problems. That is why using them should not be an option if you can super-filter your pool water beforehand using a pre-filter.

Here are few chemicals you can use to treat your pool water to avoid staining pool parts and discoloring your water. The first chemical I will recommend is the one that absorbs metals from pool water and is placed at the skimmer basket or pump basket, and the second one suspends/holds heavy metals in a solution, and last one removes fresh metal through the filter.

A. CuLator Ultra PowerPak 4.0

This is my second recommendation after a pre-filter since chemicals are not added directly into the swimming pool, but I use it on skimmers or pump baskets. CuLator Ultra PowerPak is capable of removing 4.0 ppm total dissolved metals from 20,000 gallons pool water. This product can be used for up to 30 days or longer, depending on the amount of metals in your water. It has an added advantage since you can use it to remove several metals, such as copper, iron, manganese, silver, cobalt and nickel, and can be used successfully on fresh or salty water.

B. Sequestering Agents

Metal sequestrants work differently by preventing metals from coming out of a solution inside a swimming pool. By preventing metals from separating in a solution, pool parts will not be discolored and water colour will not change.

When choosing a sequestering or chelating agent to use in your pool, you have to be careful. Most of the sequestrants in the market have a significant amount of phosphate as one of the major components, and you don't need a bunch of phosphate accumulating in your pool due to high chances of scaling inside and around the pool.

The only sequestering agent I know about without phosphate is one that I have been using for a while now: the SC-1000 by Orenda Technologies. Besides preventing metal stains and pool water discoloration, the SC-1000 is also capable of removing and preventing calcium scaling, since it controls calcium levels in pool water.

C. Metal and Metal Stain Remover

To remove fresh metals through the filter, you will need Metal Magic by Pro Team that also clears metal stains. Metal Magic is the best option compared to metal sequstrant since it practically removes fresh metals in your pool water via your filter. This is also the best option after CuLator ultra power pack and is used on a monthly basis for preventive measures to avoid metal stains.

Additional Information

If you need more detailed information, check out my other article on How to Remove and Prevent Metal Stains in a Swimming Pool.

Additionally, included below is a detailed video done by River Bend pool company explaining how to use SC-1000 to control metals and scales in your pool.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

Questions & Answers

  • I filled my small pool with well water and creek water. When I shocked it, it turned a light green. My pool store guy says there’s no turning back and I have to drain the pool and refill it and put an iron additive to the water. I can’t believe I can’t fix this without draining the pool. What if this was a 30,000-gallon inground pool?

    You have Copper metal in your pool and that is Copper metal stain that is light green in colour. Reduce the pH to 7.2 and leave free chlorine(FC) to deplete by itself to 0ppm. You may have to drain your pool water partially to reduce FC to zero faster.

    Add Polyquat 60 Algaecide by Pro Team that is metal free to avoid algae while FC is zero. Add ascorbic acid to your pool, 1 pound of ascorbic acid for every 10,000 gallons and metal stains should disappear. Add Metal Magic by Pro Team as directed for initial dose to remove all fresh metals in your pool water via the filter, use Metal Magic after every 1 month to remove fresh metals and avoid more metal staining in the future. Here is more on how to remove metal stains and prevent metal staining in the future: https://dengarden.com/swimming-pools/Fixing-Swimmi...

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