How to Clear and Prevent Metal stains in a Swimming Pool

Last Updated: | By Barack James


How to Remove and Prevent Metal Stains in a Swimming Pool

Why does my Pool Discolor when I Add Chlorine? 

Swimming pool fill-water sourced from the Well is always full of heavy metals such as Copper, Iron, Silver, Magnesium, or Manganese. These metals can also be found in some untreated municipal water, and to avoid metal stains, it's always advisable to test your fill-water for heavy metals before using any water in your pool. 

Heavy metals when oxidized by chlorine, produce metal stains in a swimming pool. If your pool water has been changing color to green, brown, or black especially after adding chlorine or when pH is high, that is a metal stain and it's caused by the presence of heavy metals in pool water commonly Copper, Iron, and Silver. 

The good news is that you can get rid of the metal stains and prevent metal staining in the future. The most common types of metals you can find in your pool water are Copper and Iron. However; Silver, Magnesium, and Manganese may also be found in some pools especially when using Well fill-water around regions where these heavy metals are mined.

Copper metal will be oxidized by Chlorine or high pH producing light green stains; Silver is oxidized producing black stains; Iron is oxidized producing brown or rusty stains, Manganese and Magnesium are oxidized producing purple stains that can be seen on the walls, floor, liners, fiberglass, and sometimes on pool equipment.

However, you need to be sure your pool is affected by metal stains, which is inorganic stain, and not organic stain from algae and other organic matters that may cause stain colors similar to metal stains. This article is divided into 3 main parts: 

  1. Test to determine the cause of stains(organic or inorganic stain)
  2. How to clear metal stains and
  3. Ways to prevent metal stains in the future 

1.) Is it Organic Stain or Metal Stains?

If not careful, you might mistake Copper and Silver stains to be green and black algae respectively. Before starting treatment for metal stains, you need to be certain that you are dealing with metal stain (inorganic stain) caused by heavy metals such as Iron, Copper, and Silver, or organic stain caused by algae, plants, leaves, and more.

1. Use Ascorbic Acid Test for Metal Stains

Doing this won't take your 10 minutes. To know whether it's a metal stain or organic stain and perform the right treatment; use ascorbic acid powder (or vitamin C tablet) to test the stains. Rub ascorbic acid in stain affected area, if the stains come out or loosen after applying ascorbic acid, you are dealing with metal stains and not stains caused by algae or other organic matters.

2. Use Chlorine Tablet (Trichlor) Test for Organic Stains

If you can't find ascorbic acid to do the test, you can use chlorine table (trichlor) to determine if it's an organic stain: Place a chlorine tablet on the stain for a few minutes, if the stain lightens and clears, you are dealing with an organic stain, which you can remove by raising free chlorine (FC) level to a given shock level that can range from 10ppm going upwards. 

3. Perform Overnight Chlorine Lose Test (OCLT).

Another test you can do to know what you are dealing with is the Overnight Chlorine Loss Test(OCLT): Copper stain is light green but might look like green algae. To do the test, chlorinate your pool as required in the evening when the sun is gone and take Free Chlorine(FC) reading both in the evening of chlorination and in the following morning, if you lose 0-1ppm of FC, it's not organic stain but metal staining; algae eat up lots of Chlorine and you will lose above 1ppm of FC overnight.

2.) How to Clear Metal Stains in a Swimming Pool

When you are finally sure it's metal staining, you can use ascorbic acid to remove the metal stains from your pool. Follow the guide below to clear metal stains using ascorbic acid. 

Step 1: Lower FC Level to 0ppm

Reduce FC level to 0ppm. Reducing your FC is essential to avoid more metal stains while eliminating the stains. Also, the chlorine will eat up most of your ascorbic acid ending up using a lot to clear all the metal stains.

To lower your FC level, you can use chlorine neutralizing agent or drain and refill a portion of your pool water, or wait until FC gets 0ppm naturally. However, waiting for the FC level to drop by itself will take time and you have to add polyquat 60 algaecide (non-metal algaecide) in your water to prevent algae while the FC level is at 0ppm. 

Step 2: Lower pH Level to 7.2

Ascorbic acid will be ineffective in pH above 7.2, consequently, reduce your pH to 7.2 if higher than that. High pH also causes metal stains and you don't want more stains while clearing the stains.

You can use pH minus or muriatic acid to lower pH, but remember muriatic acid will lower your alkalinity too and you will have to balance your water when done. 

Step 3: Put your Filter on Circulation 

You need to put your on circulation so that you speed up the process by allowing water to move around the pool. This will also enable ascorbic acid to work effectively in clearing the stains without letting some out of the pool through the filter.  

Step 4: Add Correct Amount of Ascorbic Acid 

Add 1 POUND of Ascorbic acid to every 10, 000 GALLONS pool; such that if you have a 40k gallons pool, you will add 4lbs or pounds in your pool targeting most stains affected areas.

Put the correct amount of ascorbic acid for your pool size in a cup and drop it down the sides all around your pool, and ascorbic acid to circulate for 30 minutes and watch stains disappear slowly until you see no more stains. 

If the stains are not all gone, add more ascorbic acid on the spots where you can still see traces of stains and repeat until you see no more stain. 

Step 5: Add Metal Sequestering Agent  

When your pool is free of metal stains, remember that your water is still full of heavy metal compounds and will stain again the moment you add chlorine. To avoid more metal stains, you need to add a metal sequestering agent, which is a chemical that holds together heavy metals like Iron and Copper in a solution so that they don't get oxidized by chlorine to produce metal stains. 

Step 6: Put your Filter on FILTER and Balance your Water Chemistry

After clearing all the stains and adding a metal sequestering agent, put your filter on Filter and leave your pool for 24 hours before adding anything. After 24 hours, you can start balancing chemicals.

Balance pH and TA Levels: Ascorbic acid though not a strong acid, will lower your pH and total alkalinity (TA) a little lower than you left them, however, the difference might be very small. Test your pH and TA and make adjustments as appropriate.

If TA is balanced but you still need to raise the pH, use Borate to raise your pH without raising the TA level further but don't let pH go higher than 7.2 to keep metal stains in control. 

Bring up Free Chlorine Level: After getting your pH and TA in proper balance, you can start raising your FC level. I prefer you use liquid chlorine that will not raise your pH, Calcium, and Cyanuric acid level that is caused by powder and tablet chlorine.

You need to chlorine slow by slow while watching on traces of metal stains. At first, raise your FC to 2ppm: If you see no stains in 15 minutes, go ahead and raise your free chlorine to the recommended level for your Cyanuric acid level in the water. Here is more on how to shock your swimming pool whether saltwater or non-saltwater. 

However, if you see stains while adding chlorine, stop adding more chlorine and make sure your pH is at 7.2 before adding more metal sequestering agents to lift the stains. Don't shock your pool for 2 weeks and leave ascorbic acid and metal sequestering agent to work fully.

After 2 weeks, your chlorine should now hold easily since ascorbic acid has been used up without causing more stains. Remember to use metal sequestering agent treatment frequently as directed by the manufacturer, usually doses of one week to one month to control metal stains. 

3.) Way to Remove Metals in your Pool Water

Removing metals completely out of your pool fill-water or inside your pool is the easiest and most economic way to prevent metal staining.


To be sure your pool is free of heavy metals all the time, use Lamotte ColorQ Pro 11, which is a digital pool water test kit that takes the reading of all pool chemicals and most common heavy metals including Copper and Iron. 


Below are 4 ways and products you need to completely remove heavy metals in your swimming pool fill-water before entering your pool, or remove metals through your pool filter:

        1. Filter Well Water Before Entering your Pool

This may seem lots of work and expensive but is the easiest and most economic way to control metal stains in your pool. To filter metals out of your fill-water before entering your pool, use Pleatco Disposable Pre-filter to get rid of metal compounds before entering your pool system. Pleatco pre-filter fits in any standard garden hose and can filter up to one-time water capacity of 40,000 gallons.

       2. Add Metal Remover in your Pool Water

You can do away with heavy metals inside your pool by adding ProTeam Metal Magic in your water. ProTeam Metal Magic is the best metal, metal stain, and scale remover in the market. It removes all metals including Copper, Iron, Silver, and Manganese through the filter. For better results, it's used in monthly doses and can be the best option if you need the fastest way to remove metals from your pool water to avoid stains. 

       3. USE CuLator Ultra PowerPak 4.0


Just like pre-filter, CuLator Ultra PowerPak works by removing metals from your water before entering the pool and therefore will not be oxidized when chlorine is added.

CuLator can be used on the skimmer or pump basket and has an added advantage since it can remove 4.0ppm total dissolved metals like Copper, Iron, Zinc, and Manganese in 20,000 gallons of water, and you can increase its metal filtering power by putting more in both the skimmer and pump basket.

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