Types of Pool Stains

Types of Pool Stains ~ All year-round, your pool interacts with an overwhelming amount of matter. You have chemicals, sweat, and oil of swimmers, algae, or even urine! It’s imperative to know at least basic pool maintenance, and that includes cleaning pool stains as well. 

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Pool stain treatments are not just about the method. You also have to consider the type of stains you have. Classifying them is important in order to give the optimal pool treatment. 

However, identifying pool stains can be a piece of work sometimes. They do look deceptively identical in some cases. 

That said, here’s a rundown on the two main types of pool stains—organic and inorganic.

Organic Stains 

Organic pool stains come from living matter, commonly from the following: 

  • Bugs
  • Algae
  • Plants
  • Humans 

Leaf Tannins 

Sometimes, leaf tannins start to dissipate when leaves sit still long enough on or beneath the surface of the pool. They basically discolor the pool and usually appear in brown or green stains. 

Leaf tannins are most prominent during the off-season, especially when the pool is being winterized. They can also hang around your skimmer baskets. Imagine a teabag slowly dispersing into hot water—that can also happen with leaves. 

To get rid of leaf tannins seeping into your pool, a simple chlorine shock usually does the trick. 

Green Algae

The easiest to appear and the easiest to get rid of. Green algae thrive in neglected pools and can be easily killed with algaecides or a pool shock. 

After treating algae, expect to see some dark discolorations on the surface of your pool. They usually go away on their own but you can get rid of them quickly with some additional shocking. 

Yellow Algae 

Careful with this one. Yellow algae, also commonly referred to as ‘mustard algae’, can be easily mistaken for normal dirt or debris due to their dark yellow color. 

You can simply use a flocculant to clump together the algae. Taking this step helps for easy brushing afterward. 

Yellow algae can be very stubborn, though. They can cling tightly to pool walls and even pool floats and equipment. Hence, you’ll need to vacuum the pool in order to really deep clean the perimeter. 

It also helps to know that yellow algae is highly tolerant to chlorine. Meaning, algaecides or pool shocks are no use to yellow algae. 

Black Algae 

They have a black to purplish hue and can be commonly found in the steps or shady areas of concrete pools. By far, they are the most stubborn of all organic stains. 

Getting rid of black algae requires a combination of religious scrubbing and intense pool shocks. Sometimes, you’ll even need a floc and vacuum just to completely wear them down. It’s a lot of work, so be ready for it.

Pink Algae

Pink algae is not exactly algae, rather a slimy bacteria that can be commonly found in smooth surfaces of the pool. 

They thrive in shaded and quiet areas of the pool. Also referred to as ‘pink slime’, pink algae can thrive and are chlorine-resistant. 

To get rid of pink algae, you can just simply brush them up the water surface. Also, it goes without saying, but having proper pool chemistry is vital to prevent them from growing. 

Iron Bacteria

Besides pink algae, there’s a different form of bacteria that can feed on your pool iron. They give off a yellow-brownish film that can easily be mistaken for yellow algae.

The thing with iron bacteria is they can look like iron stains as well. Hence, if you treat them like iron stains by merely brushing them off, the bacteria will still stay intact and will cause discoloration soon after you chlorinate your pool. 

Believe it or not, this causes confusion (and frustration) for many novice pool owners, which is why it’s very important to properly identify the stain before treating it. 

Liner Mold 

If you have a vinyl pool, chances are, you’ve dealt with a liner mold, or at least seen one. Liner mold grows under the liner due to untreated soil. 

As harmless liner molds are, they do look yucky! The best way to get rid of them is to treat the soil underneath the liner.

How Do You Know If It’s An Organic Stain? 

Double-checking for an organic stain is simple. Use your trichlor puck by placing it on the stain. Remember that organic stains react positively to certain chemicals. If the stain is slowly vanishing, then it’s an organic stain. 

You can also try this test with granular chlorine in place of the trichlor puck. Simply scrub the stain after drizzling on some granular chlorine. If the stain fades, you have yourself an organic stain. 

Inorganic Stains

These kinds of stains come from metals, rusts, and even minerals. Inorganic stains cannot be brushed off and need to be treated with more sophistication. 


Iron stains harbor rust, brown-orange color. They are usually caused by any of the following: 

  • Well water
  • Municipal water
  • Fertilizers
  • Pesticides
  • Herbicides

Unsure if it’s an iron stain? Do the Vitamin C test. Simply grab 15 to 20 vitamin c tablets, put them in a permeable bag (ex: sock), then crush them altogether. 

Hold the bag near the stain for about 3-5 minutes. If it begins to slowly fade, it’s iron. 

You can deal with iron stains using a polyfill pillow stuffings. Simply place the polyfill stuffings onto the skimmer basket then let it do its magic. 


Copper stains come in an array of colors ranging from turquoise to grey and black. Below are some of the usual suspects for copper staining: 

  • Mineral systems
  • Ionizers
  • ‘Natural’ sanitizers
  • Algaecides containing copper
  • Copper plumbing
  • Low pH
  • Copper plumbing

In order to remove copper stains, get a sock filled with dry acid. Then, rub it on the stain. That usually does the trick for small stains. 

For bigger stains, a non-drain acid wash is recommended. 


On their early stages, cobalt stains appear as small, black spots. Soon after, they grow fingers like stars. They are caused by high chlorine and pH levels that have lingered for too long. Old surfaces can also be the source of cobalt stains. 

If your pool has an old surface, you need to treat them immediately and vigorously. Cobalt stains are stubborn and tend to resurface quickly. 

Calcium Buildup 

Calcium buildup comes in the form of white or off-white deposits that cling on to your tile grouts, pavers, or concrete. They are caused by thrown-off water chemistry, usually by imbalanced pH levels. 

Although they may look harmless, calcium scales can do serious damage to your pool structure and system. Let them sit uncared for and the following things will happen: 

  • Clogged pool filter
  • Coated pipes
  • Crystallized calcium buildup 

Never underestimate the presence of calcium buildup. The longer you leave them alone, the harder they are to scrape off. They can end up leaving your pool system all wrecked and malfunctioning. 

Thankfully, you have muriatic acid to combat these stubborn minerals. Muriatic acid is the most effective form of treatment for calcium stains. However, be careful when putting them to use.

Always remember to wear protection such as gloves, goggles, and clothes with ample coverage to prevent burning and skin damage. 

Wrap Up 

Swimming pool stains come in different colors, shapes, and sizes. The main thing you have to remember is this: 

Organic stains = usually resolved through brushing

Inorganic or metal stains = usually resolved through chemicals

Fiberglass pools also make it easier for you to remove stains better than concrete because of their smooth surface. If you happen to own a fiberglass pool, you’re already ahead of the game.

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