Pool chemistry: A Complete Guide for Pool Owners


So, why does pool chemistry matter?

If you don’t keep your pool chemistry balanced, it leads to various problems, such as bacteria, viruses, and algae growth.

Not only do these problems pose a threat to your health, but they can also be lethal in rare cases.

But no worries. It’s not too difficult to learn to manage your pool chemistry, and this guide will hopefully serve as a great starting point for you.

Most important terms for pool chemistry

Let’s first start by discussing the most important terms you need to know for pool chemistry. 

Free chlorine: It is a chemical that works as a sanitizer that keeps the pool safe and free from germs. It breaks down harmful microorganisms in your pool. Free chlorine can be consumed in two major ways. It is used up when breaking down organic matter like algae, or the sun can burn it off.

This means you’ll want to constantly replenish free chlorine unless your pool has a saltwater system, which automatically generates chlorine.

pH: It’s a measure of how acidic or basic your swimming pool is. If the pH level gets too low in your pool, it can burn your eyes and skin. If the level gets too high, it can corrode metal parts in your swimming pool. 

The ideal pH range is between 7.4 to 7.8. You can lower it by adding an acid and you can raise it by adding soda ash.

TA (total alkalinity): It buffers the change to pH in your swimming pool. If you have a low TA, your pH level is going to fluctuate a lot. If your TA level is very high, your pH is going to drive up very quickly. The sweet spot for total alkalinity is between 70 and 90 ppm.

Calcium hardness: Measures the level of calcium in your pool water. 

If you have a plaster or gunite pool and don’t have any calcium in your water, the water is going to dissolve the calcium out of the walls of the plaster. So, it’s important to maintain a certain level of calcium if you have a plaster or gunite pool. 

That said, if you have too much calcium in the water, you can end up with calcium scaling, which is tough to clean up. 

The ideal level for the calcium hardness is around 250 to 350 ppm, assuming you own a plaster or gunite pool.

If you own a vinyl pool, there’s no need to have calcium in your pool. But, having some calcium in your pool won’t hurt anything as long as the level stays below 350 ppm.

CYA (cyanuric acid): Acts as a sunscreen for the free chlorine in your pool. CYA prevents free chlorine from being burnt off by the sun. 

But too much of CYA will lower the effectiveness of free chlorine, so you want to keep the level at an ideal range.

If you have a saltwater generator, the CYA’s recommended level will be between 60 to 80 ppm. If you don’t have a saltwater generator, you want the level to be between 30 to 50 ppm. 

The only way to lower CYA is by partially or fully replacing your pool water. Unfortunately, you can’t use any chemicals to break down CYA and lower its level. 

Ideal chemical levels for your pool

You should maintain the free chlorine level at a minimum of 1 part per million (ppm) for pools. For spas, it should be a minimum of 2 ppm.

If you use a chlorine product with cyanuric acids, such as trichlor, you want to keep the free chlorine level slightly higher at 2 ppm in a pool and 3 ppm in a spa.

Total bromine needs to be at least 3 ppm for pools and 4 ppm for spas.

Spraygrounds and special features must maintain a minimum of 2 ppm free chlorine and 4 ppm total bromine.

If you use chlorine disinfectant, then you need to test the combined chlorine level as well.

The combined chlorine results from free chlorine reacting with organic compounds containing nitrogen, such as urine, sweat, and environmental contaminants. 

The combined chlorine level should not exceed 1 ppm. A high level may result in a strong chlorine odor.

Pool shocking refers to adding a large dose of chlorine to the pool or spa at one time. The dose should be 10 times the amount of combined chlorine minus the existing free chlorine to achieve breakpoint chlorination. This will eliminate combined chlorine from your pool and increase the free chlorine level.

Shocking should only be performed when the pool is closed and in the evening when people have left to allow the disinfectants to circulate properly.

How often should you measure the chemicals in the pool?

Most important chemicals to measure in your pool                                                                                 

You will want to check your pool’s pH and chlorine level at least once a week in the summertime.

An alkalinity level is quite stable and doesn’t change that much if your pool is older. So you can check your pool’s alkalinity about once a month.

If you own a new plaster pool, you’ll want to check the alkalinity level more frequently at around two to three times a month since your pool is more susceptible to a change in alkalinity level.

Calcium hardness rarely needs to be checked since it barely ever changes. It suffices to check the level once every 3 months or so.

Cyanuric acid (CYA), also known as a pool conditioner, protects the chlorine in your pool. The pool industry has developed cyanuric acid to help stabilize chlorine and protect chlorine from light degradation. 

You’ll want to check your pool’s CYA level at the beginning of the pool season, which is around March. You want to ensure that the level is anywhere between 30 and 80 parts per million.

If the CYA level is really high at over 100 or 150 parts per million, you’ll want to drain a little bit of your pool water to bring the level down. This is necessary since an overly high CYA level will render chlorine ineffective. Just make sure your pool water is clean before draining.

Other chemicals to keep track

If you have a saltwater pool, you will want to check the salt level at the beginning of the season. You may also want to check the level a few months into the season. Although the salt will not evaporate, it can potentially splash out through backwashing, which can cause the salt level to drop.

There are other chemicals in your pool, such as phosphates and nitrates. But those are generally not a problem in most areas. 

That said, if your pool is not holding chlorine week-to-week, and you notice a lot of algae growth, there’s a slight chance that may be due to a phosphate problem.

If you suspect the phosphate level in your pool is off, you can run the test with a phosphate test strip, or you can take your pool sample to your local pool store, and they’ll check the phosphate level for you. The phosphate removers are effective for reducing the phosphate level in the pool.

Chlorine for pools 

So why is chlorine essential for your pool? It’s the sanitizer that kills bacteria, viruses and keeps the water safe to swim in.

Chlorine is the most effective chemical for killing off harmful microorganisms in your pool water. So, you must maintain the chlorine level at an ideal range.

Typically, you want to keep the chlorine level at around 3 parts per million (ppm) along with a pH level of about 7.4.

If your pool has a low chlorine level, and your pool fails to kill bacteria and virus, you are much more likely to get an infection. 

In some cases, it’s okay to keep the chlorine level at one ppm if you have an ionizer or UV that will also help eliminate bacteria and kill viruses. 

That said, they are not effective for eliminating certain types of bacteria and viruses. Chlorine is the only chemical that can reliably kill most harmful organisms in your pool.

As a last note, it’s vital to keep the pH level balanced in your pool.

If the pH level is really high, chlorine will not be as effective, and If the pH is low, the chlorine will get depleted faster.

7 Common types of chlorine used to chlorinate pool water

1. Gas chlorine

It fell out of favor in the last decades. It has a low pH level and is dangerous with a low staying power. Additionally, you need an expensive chemical feeder to feed it into the plumbing continually.

2. Cal-Hypo

Calcium hypochlorite is the oldest form of chlorine, and it is the strongest shock available. It’s got a high pH level and typically found in granular forms, but it also comes in tablet forms. It’s highly volatile and reacts with lots of other things.

On top of raising the chlorine level, it will also cause the pH and calcium hardness level in your pool to rise. 

Cal-Hypo’s high calcium content can cause cloudy water, so you need to keep a close watch on your pool chemical level when adding it to your pool.

3. Liquid chlorine

Liquid chlorine is the purest form of chlorine. It’s a great way to maintain your pool chlorine level without affecting the cyanuric acid level since the liquid chlorine doesn’t contain any cyanuric acid. It’s a good choice if your pool already has a sufficient level of cyanuric acid.

4. Clorox bleach 

Clorox bleach is the same as liquid chlorine, but just slightly weaker. The chemical has a high pH level, and it is fast-acting but with a low staying power. It’s great for shocking and bringing the chlorine levels up quickly.

5. Trichlor

This is a chlorine product that’s most commonly found in 3-inch or 1-inch tablet form. You can also get it in a crushed-up, granular form. 

Trichlor is a low pH form of chlorine that contains a fair amount of cyanuric acid. In fact, about half the weight of a trichlor tablet consists of cyanuric acid. 

So if you primarily use trichlor tablets to chlorinate your pool for the entire summer, the cyanuric acid level in your pool may end up quite high. Sometimes, the level can surpass over 150 or 200 parts per million, depending on how many tablets you put in every week.

It’s got great staying power, especially in tablet form. For this reason, it is ideal for the hot environment as it will maintain a balanced level of chlorine throughout the week.

Also, trichlor tablets have minimal effect on pH, so it’s a good choice if you don’t want to disrupt your pool pH. 

6. Dichlor

Dichlor is more of a pH neutral, granular form of chlorine that tends to raise the cyanuric acid level significantly higher than trichlor. It works great for starting up a pool or where pH needs to be very balanced, such as a tile or a fiberglass pool.

7. Lithium

Lithium is the newest form of chlorine in the market and comes in a granular form. It dissolves quickly in your pool water and doesn’t affect the pH level, and it’s safe on just about all pool and hot tub surfaces. The only downside is that it is very expensive.

Mineral pools to reduce chlorine

Mineral pools are the only types of pools where you can safely reduce the chlorine concentration. NASA used mineral water to purify drinking water in space during the Apollo program.

Mineral pools cost less than salt water or traditional chlorine pools. So if you are installing a new swimming pool, then you may consider mineral pools.

Alternative to chlorine

Alternatively, you can use ions to sanitize your pool. 

A silver ion adheres to the bacteria’s membrane and forces the membrane to alter itself, spilling out its content into the pool.

One free chlorine is only effective for dealing with one bacteria. On the other hand, copper or silver ions will destroy bacteria, and those same ions will continue to kill other bacteria. So it’s not a one-trick pony. It will continue to fight bacteria over and over again. 

That’s why ionized pools are so useful because you don’t need much ions to keep your pool safe. That said, some believe that ions are not as effective as chlorine for fighting off bacteria and pathogens.

Many people mistakenly believe that a saltwater pool is a healthier alternative to a standard chlorine pool, but this is just a myth. Salt chlorinator converts salt in the water into liquid chlorine. This means there’s no difference between the saltwater pool and a regular pool treated with chlorine. The end result is exactly the same.

How to change pH and alkalinity in your pool

Chemicals for altering the pH in your pool

pH is logarithmic, which means the distilled water with a neutral pH of 7.0 is 10 times more acidic than the saltwater with a pH of 8.0. And a soft drink like Mountain dew, with a pH of 3.0, is 10,000 times more acidic than distilled water.

The pH scale is a measure of the positive hydrogen ion that is loose in the water. In other words, it measures free-floating protons. The more hydrogen ions you have in the water, the lower the pH, and the more acidic it is. The more hydroxide ions you have, OH-, the more alkaline, the more basic your pool water is.

In swimming pools, there are generally two different types of acid you can use to lower the pH level: Muriatic acid and sodium bisulfate.

Muriatic acid is a dangerous industrial chemical, and you have to be a trained professional to use it, or you could get very hurt. You can burn your skin if it comes in contact with your body. And you can even burn your lung if you inhale it. 

Sodium bisulfate, also known as dry acid, is a safer alternative to muriatic acid. That said, this chemical compound can also be dangerous in the wrong hand who’s not experienced.

Acids release hydrogen ions and react quickly. If you have strong acids in your pool, they’ll react to completion. In other words, once acids react with something, those forms are set unless something else comes in and reacts with them.

If you need to increase your pool’s pH level, you can use borax, which is a laundry detergent enhancer. It raises the pH level rapidly without raising the alkalinity level much. Soda ash effectively raises the pH level, but it will also slightly raise the alkalinity level.

Although baking soda raises the pH level slightly, it is probably the least effective way to raise pH since you’ll need to use a lot of them to notice any difference. That said, it’s a great chemical for raising alkalinity without affecting your pool pH.

There are a few factors that can cause pH to rise on its own. For example, if you have a salt-chlorine generator, the separation of the sodium from the chloride releases hydroxide ions, causing the pH level to increase.

Chemicals for altering the alkalinity

Alkalinity, also known as a buffer, will react and neutralize the acid. You typically want the alkalinity to stay within the range of 80 to 100 ppm. But the ideal range can vary based on other variables, such as what part of the country you’re in.

Simply put, alkalinity is a measure of the water’s resistance to pH change, measured in parts per million. 

Baking soda, also known as sodium bicarbonate, is used to increase the alkalinity in pools. It reacts with hydrogen ions in the water but doesn’t react with plasters, metals, and other pool parts. This makes it very safe to use for pools.

Baking soda will effectively raise the alkalinity without raising the pH.

There are several acid products you can use to reduce the alkalinity level in your pool. These acid products include muriatic acid, dry acid, No Mor Acid, and Acid Magic.

I suggest you use muriatic acid if your pool’s alkalinity is high. Dry acid (sodium bisulfate) is just as effective, but it will unnecessarily increase the sulfate level in your pool.

If the pH level dips below 7.0, stop adding acid, let the pH rise by itself, and start adding again.

So, with a high alkalinity level in your pool, it’s sufficient just to add muriatic acid every day. Eventually, your pool’s alkalinity will decrease to an acceptable range, which is between 80 to 120 ppm.

pH and sanitizers in your pool

The lower the pH, the more effective and aggressive chlorine is at sanitizing and oxidizing organic compounds.

Chlorine is 21% effective at a pH of 8.0, but it’s nearly 70% more effective at a pH of 7.

This is why when you are shocking the pool; you always want to add a little bit of acid to drive the pH down so that chlorine can work better.

So why not just lower the pH level even further below 7.0 and use less chlorine? 

The ideal pH for the human body, blood, mucous membrane, and eyes is around 7.4. 

So if your eyes start to burn when you swim in your pool, it’s not because of chlorine. You are experiencing discomfort because the pH level is significantly higher or lower than 7.4. 

As a side note, the pH level of a spa or hot tub should run slightly higher towards the 7.6~7.8 range because when people enter the hot tub to bathe, the pH level tends to fall a little.

Testing pool chemistry

Different test kits for pool chemistry

I recommend two good test kits: The Taylor K 2006 and a basic test kit with the OTO solution. 

You can use the Taylor K 2006 as a DPT test. It’s a useful test kit, and all the reagents should last you the whole season. You can also get the basic test kit with the OTO solution to test the chlorine. The test kit also comes with a phenol red, which you can use to test the pH level.

You don’t want to rely exclusively on test strips since they are just estimates in most cases. So I recommend you invest in a good reagent test kit.

The color queue pro 7 is another reliable test kit that is slightly on the pricey side. It uses a photometer to read the colors and gives you a digital reading on the levels. It comes with seven test factors. This is a useful kit if you have trouble reading the reagents.

For most people, I’d recommend using a DPD test kit to test your pool chemicals. The following instructions assume you are using the DPD test kit to carry out your testing.

Testing pool pH

To test the pH level, you need to rinse and fill the comparator and then add the pH reagent according to the test kit’s instruction. Do not hold the bottles at an angle when adding the reagents.

Next, cap the tube and invert it to the mix.

Now, match the color in the tube to the color standard on the chart using natural light or a white surface background to achieve an accurate color reading. Record the outcome as the pH unit.

Testing alkalinity

Alkalinity should be maintained at 60 ppm or above and tested at least once per day.

To test alkalinity level:

  1. Rinse and fill the comparator tube and add the reagents according to the instruction on your test kits.
  2. Count the reagents’ drops until the color change is visible and multiply the number of drops by 10.
  3. Record it as ppm alkalinity.

Testing cyanuric acid

Cyanuric acid is often used as a stabilizing agent in Tri-Chlor and Di-Chlor disinfectants. You can also add it separately to help chlorine work better. 

While cyanuric acid is most effective between 30 and 50 ppm, it cannot exceed 70 ppm by State law.

Unlike chlorine, cyanuric acid does not breakdown or evaporate, so it will continually build up in the water over time.

To test cyanuric acid, rinse and fill the dispensing bottle with equal parts water to be tested with the cyanuric acid, and mix it for 30 seconds. 

Next, slowly transfer the solution to a small comparator tube until the black dot on the bottom disappears when viewed from the top. Then, read the liquid level on the comparator block.

The pool temperature must not exceed 90 F, and spa temperature must not exceed 104 F. At higher temperatures, the disinfectant can quickly evaporate.

Dos and Donts of chemical safety

1. When storing chemicals, keep the lids tight, and make sure they are stored in a dry place away from other chemicals like acids and algaecides.

2. Fumes can build up inside the buckets and chemical feeders, so take precautions when opening up buckets, and make sure you have some ventilation.

3. Don’t ever mix different forms of chlorine or mix chlorine with acids. The result can be deadly.

4. You can directly add most forms of chlorine to your swimming pool.

But in a powder form, you want to make sure that you pour it low and close to the water in case it gets carried by the wind. This will help you avoid getting it on things where you don’t want. 

You also want to avoid applying the chlorine into your pool while someone’s in the pool.

5. For sensitive surfaces like viny liner pools, you can premix your chlorine in a bucket to dilute it before pouring it into your pool so it doesn’t damage or stain your vinyl.

6. When it comes to chlorine tablets, there are only two ways to dispense them in the swimming pool.

One is through a chlorine floater if used in a residential setting. In a commercial setting, you want to put tablets into an inline chlorine feeder. 

You want to avoid putting tablets directly into the pool, a skimmer, or a pump basket. Otherwise, you may end up damaging your pool equipment.

7. You never want to add more than a half-gallon of acid at a time to an average-sized swimming pool.

When working with acid, you can pre-dilute it by pouring acid into a bucket partially filled with water. Never pour water into acid. You can also wear a mask as a further precaution. 

Pour down to the side of the container to prevent splash out and stand upwind. Be careful not to spill or splash on the deck or concrete, as acids can cause damage.

Green pool chemistry

If your pool turns green, you want to apply enough chlorine into your pool water to kill organic matter. 

You can use algaecide, such as sodium bromide, to supplement the chlorine with pool sanitization. Bromine is well known to be the killer of algae, and it is often used in hot tubs, resorts, hotels, etc.

You should be able to effectively treat algae using a combination of chlorine and bromine-based algaecide.

In sum, you’ll want to raise the chemical levels of sanitizers and have your pool circulating and running with a clean filter for a prolonged period. This will clear up your pool and restore the chemical balance.

There’s no need to test the pH, the alkalinity, and the cyanuric acid when you begin the green pool cleanup process. 

Since the pool is basically a swamp initially, you want to add the chemicals, get your pool blue again, and then balance the water.

You may want to reconsider if you were thinking about draining your dirty pool water. In most cities, draining a green pool into the street is a violation of up to a few thousand dollars fine.

In case the cyanuric acid level of your pool is high, the only real way to treat that is to drain the pool. But you will want to make sure that your pool is clear before you ever consider draining the water down.


Although learning about pool chemistry can appear intimidating initially, it’s not that bad when you put in a little bit of time and effort.

Even if you don’t manage the pool yourself, it’s always good to have a basic understanding of what’s going on inside your pool, so you know what needs to be done to improve your pool’s condition.

Anyway, I hope this article helped you understand pool chemistry better!

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