Why a Swimming Pool Turns Green After Adding Chlorine

Updated on March 15, 2019
Barack James profile image

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

There are a couple of reasons why a swimming pool might turn green. One of the major reasons that all pool owners know is algae breakout. Another reason that very few pool owners know about is that when heavy metals, specifically copper, react with chlorine, a green solution known as copper II chloride is formed. This occurs the moment copper is oxidized by chlorine and comes out of the solution forming discolored water or pool parts. As such, it is true to say that all heavy metals dissolve in water and are not visible until they get out of the solution through oxidization.

To be sure that the greenish colour in pool water is not algae, you can carry out an Overnight Chlorine Loss Test (OCLT). This test should be done at night when there is no sun light that consumes free chlorine. When a little of chlorine is used up (0-1 ppm), the OCLT result indicates that copper metal is present in the water and not green algae or any organism that could have used up a lot of chlorine (up to above 1 ppm). Additionally, algae will not turn swimming pool water green within hours and has to be noticed as it develops into a full green colour.

Causes of Copper Solution in a Swimming Pool

Copper is one of the most problematic metals in a swimming pool. It can find its way into the pool in a number of ways. When copper gets out of the solution, a green colour is released, and that is exactly what chlorine does through oxidization.

As a pool owner, you have to know how metals get into your pool. Here are the most common sources of copper solution in a swimming pool:

1. Adding Pool Chemicals With Copper

Using pool products with copper as a component (such as an algaecide) is a common way. Constant use of some algaecides will accumulate a significant amount of copper solution in a swimming pool. This may lead to pool water turning green when copper solution reacts with chlorine shock.

2. Using Fill Water From a Well

Wells are commonly known to be one of the greatest sources of metals in the pool, especially copper and iron. Iron reacts with chlorine to form iron III chloride, which is reddish in colour.

3. Using Copper Pipes and Pool Parts

During construction of a swimming pool, copper metal is frequently used in pipes and heaters. The use of copper parts requires a lot of pool care. When the pH gets too low, metals will corrode into the water, forming copper solution. That copper then comes out of the solution when chlorine is added, causing a green colour.

What Can You Do About It?

If you can't avoid using fill water from the well, using pool chemicals made with 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 them 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 for my clients. I can confidently recommend this accurate and time-saving test kit.

How to Treat Heavy Metals 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. Once you have an accurate test kit for metals and a few other chemicals we are about to discuss, your pool will never turn green 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 the second one suspends/holds heavy metals in a solution.

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 and 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.

Additional Information

Here is my initial post that covers more about treating metals and metal stains inside and around your 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.

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