How to Identify the Cause and Remove Pool Stains

Observe the color of the stain to classify the cause. Stains in the pool can appear in many different colors, and if you can identify the stain's color, you can figure out the correct way to address it. Depending on the cause of the stain, you will need a specific cleaning method for that cause.

• There are two main categories of stains based on metal or organic origins, and the colors of the stains can vary.
• Color groups include green-brown, red-blue, blue-green-black, green-brown-red, pink-red, or brown-black-purple. Try to identify which color group your stain belongs to.

Look for organic stains at the pool's bottom. These stains may be caused by leaves, berries, algae, insects, dead animals, or other organic matter, which will leave stains if they settle at the bottom. If not cleaned up immediately, they will sink and start to decompose at the pool's bottom. Luckily, organic stains are easy to clean.

• Organic stains are often green, brown, or bluish-purple. You can easily identify them if you spot organic debris like leaves settled at the pool's bottom.
• If you suspect the stain is organic, pour a bit of chlorine on it. Organic stains will dissolve easily when scrubbed with a soft-bristled brush, while metal stains won’t.

Be aware of inorganic or metal-based stains. These substances can accidentally enter the pool from well water or corrosion from copper pipes. Even a small piece of copper, like a penny, can cause a large stain when oxidized in the pool. Metals that can infiltrate the pool include rust, manganese, iron, and copper. If you spot a rust-colored stain near the pool steps, it’s likely caused by metal, and you should check for corrosion on the steps. Look for discoloration near the steps, around the drain area, and beneath the pool's inlet. Brown or dark-colored stains are typically linked to metals in the pool water.

• The metals that commonly cause pool stains include iron, manganese, and copper. Copper comes from ionization and corrosion of copper or brass pipes, leading to blue, green, teal, black, or dark purple stains. Iron originates from well water or corrosion of iron pipes and joints, causing rust-colored, gray, or bluish-brown stains. Manganese comes from well water and causes pink, dark brown, or purple stains. Calcium comes from plaster, grout, or cal-hypo shock, appearing as white crystals.
• If you see a metal-related stain, you need to identify the specific metal causing it in order to treat it correctly.
• A common cause of copper stains, such as blue-green, is improper chemical maintenance. Low pH and high chlorine levels can also corrode copper heat exchangers in the pool’s system. Maintaining water balance can prevent metal stains from forming.

Use professional cleaning services. If you want to hire a stain removal expert, use the Yellow Pages to find pool specialists or pool supply stores in your area. You will need to take a water sample to their facility to test and identify the exact concentration and type of metals affecting your pool. The expert may recommend using a special additive designed to remove metal stains.

Collect water samples correctly for testing. Use a clean cup or bottle, and invert the bottle down to the pool's bottom. Submerge the bottle fully, then turn it upright to collect the water sample. Avoid collecting samples near the return jets or surface skimmer. It’s best to collect the sample from the middle of the pool. If that’s not possible, try to get as close to the center as you can for the most accurate results.

As part of your regular water testing plan, you should perform a TOTAL METAL test. Free metal testing only measures the concentration of metals that are not suspended, while the total metal test will measure all metals present in the water sample.

Use test strips to check pool water at home. Take a water sample from the middle depth of the pool. Once you have the sample, quickly dip a test strip into the sample. Do not shake off excess water from the strip, and hold it in the air for about 15 seconds. The strip will change color, and you will need to compare the color of the strip with the color chart on the back of the bottle to get your reading. There are various test strips for different purposes, but for now, just test for pH, alkalinity, and free chlorine.

• Use test strips at least once a week. Once a month, take a water sample to a local pool store for more professional testing, especially when opening and closing the pool.

Use a liquid test kit. There are high-end test kits available, but for residential pools, all you need is to test for pH and chlorine, using a phenol red and OTO chlorine test kit. Liquid test kits provide very accurate results, but you must be able to read the colors. For example, after adding the chemicals to the water sample, the chemicals will change color, and depending on how light or dark the color is, you will compare it to the instructions on the packaging to determine the right treatment method. Keep in mind, it’s not always easy to differentiate between the shades of colors.

• OTO chlorine is the chemical used to test the total chlorine level in the water. It’s a yellow liquid you drop into the water sample. The darker the yellow, the higher the chlorine level in the pool.
• Phenol red is a red liquid used to check the pH balance. The redder the water, the higher the pH level.
• With liquid test kits, it may be hard to see the colors at the lower end of the color scale. Be sure to use a white background for a more accurate reading.

Determine if the water source is the cause of the issue. If you’re using well water to fill your pool, test the water before pumping it in. If you find high levels of metals in the water, drain about a quarter or half of the pool water and replace it with softened water. After that, circulate the water for at least 48 hours and retest. If the metal levels are still high, repeat this process.

• If the water entering the pool is fine, then metals have likely entered through corrosion. Check for signs of corrosion in all pool equipment to ensure they’re not leaking metals into the water.

Remove organic matter from the water surface. Typically, brownish-green stains are related to organic matter like algae or leaves sinking to the pool's bottom. These materials need to be cleaned up during treatment. Organic stains are generally easy to clean, but over time, the pool can start to look dull and turn light brown, making cleaning more difficult. This discoloration happens slowly, so it can be hard to notice at first.

• If you have plants around the pool, keep an eye out for fallen leaves, twigs, or fruit that might drop into the water. You can regularly use a net to remove debris from the water's surface.
• Organic debris that sinks to the pool's bottom needs to be cleaned with a vacuum. You can use a handheld vacuum or an automatic pool vacuum with sensors.

Clean the pool with acid. If you have persistent stains, you can drain the pool and clean it with acid, provided your pool has a plaster surface. This method should not be used frequently as it will remove a thin layer of plaster. To restore a bright surface, you’ll need to clean the pool with acid once every five years.

Use an enzymatic shock treatment and a stiff brush to remove stains. This method quickly removes brown-green stains that are organic in origin. You can also use an enzyme-based shock treatment for the pool. All natural enzymes used in pools will break down organic matter to eliminate stains without heavy scrubbing or the need for harsh chemicals that could damage the pool lining. If the stains are mostly concentrated around the waterline due to floating debris, the enzymatic solution will break down organic complexes and oils by accelerating chemical reactions, making the stains disappear on their own. Then, scrub thoroughly to ensure all remaining organic matter and oils are cleaned away.

Shock the pool with chlorine. Organic stains are best treated with a strong chlorine shock, followed by scrubbing with a little TLC. You can pour some chlorine directly onto the organic stain to make it disappear instantly. Remember, chlorine works very well on plaster pools, but avoid using this method on vinyl pools as it may cause the lining to peel.

• Check the water to ensure the pH and alkalinity are within the proper range. The pH should be between 7.4 and 7.6, and the alkalinity should be between 100-150 ppm (parts per million).

Remove localized stains by scrubbing with muriatic acid. Carefully pour acid into a section of PVC pipe and apply the acid directly to the stain on the pool wall. Be cautious, as if the pool is only lightly stained, you might create a prominent bright white mark against the darker background.

• After using this cleaning method, you must shock the pool to ensure the water has enough chlorine before it turns green due to algae growth.

Scrub aggressively as an alternative to using harsh chemicals. Scrub with a thin grout to seal small cracks. There are two types of brushes available on the market, one for concrete pools and the other for vinyl pools, so you need to know which one to buy. Once you’ve purchased the right brush, attach it to a pole to clean hard-to-reach stains.

Follow the proper steps to remove metal stains in the pool. The first method involves purchasing a metal cleaning product and placing it in the surface water filter basket for about a month. This is the initial step in most stain removal processes to make the pool cleaning easier later on. These products come in various sizes and strengths, so if possible, consult a pool expert to recommend the best option for your specific situation.

Turn off all machinery and devices near the pool. This means you will need to turn off the chlorine generator, metal ionizer, generator, UV system, and ozone maker. During the stain removal process and while using strong chemicals, it’s best to avoid getting pool water onto the heating system, chlorine generator, and other systems unrelated to water filtration that are close to the pool.

Lower the chlorine levels in the pool to around 0-2 ppm. The lower the chlorine, the less ascorbic acid you will need to clean the pool. You can wait for chlorine levels to drop naturally over time or after rain, but if you're short on time, you can add sodium thiosulfate to the water according to the manufacturer's instructions.

• Add algaecide to the water. Follow the instructions on the product label regarding usage and dosage for the size of your pool. This will prevent algae growth as chlorine levels decrease.

Remove metal stains using ascorbic acid. Minerals react well with this chemical, and if you believe the stains are metal-based or if the organic stain removal methods above are ineffective, try using a product that contains ascorbic acid. An easy and effective method is to crush vitamin C tablets and rub them onto the stain. After a few minutes, you will notice the stain beginning to fade. Remember that ascorbic acid works best for iron stains, while citric acid is more effective for copper stains.

• For larger stains, sprinkle ascorbic acid evenly across the pool’s surface. Start with about 250g of ascorbic acid per 40 cubic meters of water.
• Set the filter system to the "circulation" mode and turn on the pump. Let the ascorbic acid circulate in the pool for about half an hour.
• Check the pool to see if the stain has disappeared. If the stain remains, add more ascorbic acid and let the filter run for another half hour. Repeat this process until the pool is clean.

Restore the chemical balance in the pool. You need to adjust the pH, alkalinity, and hardness of the water to the appropriate range, which will be determined by the pool's size. Make sure to turn on the automatic chlorine generator, chlorine maker, UV system, and ozone generator. Place a metal filter bag in the surface water filter basket, or a larger metal filter bag in the pump basket, to maintain low metal content in the water and keep your pool clean year-round.

Establish a pool maintenance routine. Prevention will make the maintenance process much easier compared to cleaning stains. Hire an expert or use a home testing kit weekly to check the water sample for the ideal chemical balance. If you use non-tap water, be sure to check the water source, as well water containing iron may leave stains.

• Shock the pool water weekly to prevent algae outbreaks.
• Prevent issues by scrubbing the pool floor several times a week.

Prevent metal stains from returning. You must regularly check the metal content in the water because stains will return if the water contains high levels of metals. Remember, metals are naturally occurring and can enter the pool through the water supply or the corrosion of pool equipment and pipes. It’s important to monitor metal levels weekly.

• Use flocculants, also known as chelators. These substances precipitate minerals in solution, preventing free-floating minerals from creating stains. Avoid products containing phosphonic acid as they break down into phosphates, which can lead to algae growth.
• Single-use metal absorption bags can also help remove metals. Simply place them in the surface water filter basket (or pump basket), and they will absorb metals such as copper, iron, manganese, cobalt, silver, and nickel.

Prevent organic debris from entering the pool to avoid organic stains. Use a skimmer or automatic vacuum cleaner to remove leaves, berries, and twigs that fall into the pool. It's also a good idea to cover the pool during the winter when it's not in use.

• Dark stains may occur from mud washing into the pool. If you can't prevent such debris from entering, consider resurfacing the pool walls with mortar or a dark-colored liner to hide these types of stains.

• Check the pH and alkalinity weekly, and monitor the pool's TDS levels monthly or quarterly.
• Chemicals, solutions, and pool water testing kits are available at most pool supply stores.
• If you need to use a commercial stain remover, lower the chlorine level to below 1 PPM to prevent it from reacting with the product.
• For cement pools, pay special attention to pH, alkalinity, and TDS levels to avoid damaging the liner.
• After treating metal stains with ascorbic acid, you should also use a metal chelator to coagulate the metals in the water.
• You should place a metal remover product in the surface filter basket or the pump basket to absorb metals from the water.
• Ascorbic acid can remove stains. Try placing a vitamin C tablet in a sock, crushing it thoroughly, and placing it directly on the stain. You may need several vitamin C tablets, but this method is very effective.

• Natrium thiosulfate
• Algaecide
• Vitamin C tablets
• Ascorbic acid
• Enzyme-based rust prevention products
• Metal remover products

• If you are using a pool heater and notice copper stains or your water test results show high copper levels, immediately check the pH and alkalinity levels and adjust them to the appropriate range. The internal pipes of the heater, known as the "heat exchanger," are made of copper. If pH and alkalinity are not properly maintained, the heat exchanger will corrode quickly, releasing copper into the water, causing stains and damaging the heater. If the pH drops below 7.0, the water becomes acidic and begins to dissolve all surfaces and metals. Check the pH multiple times a week to prevent issues caused by low pH.