Chlorine For Pool (12 Common Questions and Answers)

Chlorine is a useful tool for ongoing pool maintenance. The application in the pool is simple, but still, there are many questions among beginners.

What is the difference between liquid chlorine and chlorine granules? What is the difference between total and free chlorine? Why is the chlorine level high and the pool still green, etc.?

In this article, I will answer the most common questions about chlorine in the pool.

The difference between total and free chlorine in a pool

Total chlorine is the sum of free (also called available) chlorine and combined (or bound) chlorine.

The difference between free and total chlorine is that “free” refers to not being combined or bonded with anything, while “total” refers to what you actually measure in pool water.  

Combined chlorine (also known as residual or bonded) refers to the amount of chlorine that has reacted with undesirable elements and can no longer be used for sanitizing purposes.

This means that it’s not active or available chlorine.

So if your pool water has 0.5 ppm of free chlorine and 1 ppm of combined chlorine, the total chlorine level would be 1.5 ppm.

Free chlorine is important because it is active and kills bacteria and viruses in the pool.

However, it cannot do anything with elements already “bonded” to another element. Combined chlorine does not go through the entire sanitization process – it’s just wasted chlorine.

The combined chlorine still has a small disinfecting effect, but this is not as good as free chlorine.

FYI: Chlorine that has already reacted forms unpleasant by-products such as chloramines. These substances are harmful to humans, and therefore it is important to constantly have free chlorine in the pool to eliminate these substances as well.

What’s the difference between liquid chlorine and chlorine granules?

Generally, we use chlorine in the form of tablets or granules for pool maintenance.

In the end, chlorine is chlorine? Why not use liquid chlorine instead?

The difference between liquid chlorine and chlorine granules is not in chlorine itself.

Each of them is a solution that contains hypochlorite acid (HOCl), which has the same effect on your pool.

The difference between liquid chlorine and granules is their ease of storage, handling, etc., while liquid chlorine requires more careful storage conditions.

But the main reason and differences are that chlorine granules are often already stabilized with cyanuric acid, and liquid chlorine is mostly not stabilized.

The liquid chlorine is often used to shock your pool water. But you could also use the liquid chlorine for your regular pool maintenance if you stabilize it with cyanuric acid.

Tip: Chlorine granules are usually applied with a chlorine feeder – an automatic device that meters the amount of chlorine put into the pool, depending on how much water goes through it.

Alternatively, you can use a cheap chlorine floater.

If you want to learn more about the different chlorine types, I recommend my blog article at the link.

Stabilized and unstabilized chlorine: What’s the difference?

A stabilized chlorine is important for ongoing pool maintenance. Otherwise, the chlorine will disassemble quickly, which means it is losing its active ingredients.

But what exactly is the difference between stabilized and unstabilized chlorine?

A stabilized chlorine contains cyanuric acid, protecting the chlorine molecule from breaking down in sunlight.

It is contained in granules or tablets (inert ingredients). It does not kill algae; only the active chlorine ingredient does this.

An unstabilized one is pure chlorine that can be added to water. But it breaks easily in sunlight, so it is recommended to use cyanuric acid-stabilized chlorine.

Here’s what you can remember:

  • We use stabilized chlorine to always have chlorine in the pool for disinfection.
  • non-stabilized chlorine is used to drive up the chlorine level in the pool. This is also called pool shock.

It is important to know that the stabilized chlorine should not be added directly to the pool water! If so, you will raise your chlorine level to a very high level and can burn your eyes.

It is necessary to add such stabilized chlorine first to a bucket with water and then gradually add it to the pool water.

How much chlorine to add to the pool the first time?

You bought an above-ground pool, and now you want to use chlorine in the pool for the first time?

Especially the cleaning during the first filling in the pool is important to have fewer difficulties later.

How much chlorine is the right amount for the first filling of the pool?

There is no actual amount of chlorine you need to use in your pool. Here, the concentration of the chlorine used is a decisive factor.

In this case, it is better to have too much chlorine than too little.

Aim for a chlorine level of 1.5 to 2 ppm for the first time.

To raise the chlorine for every 1000 gal by 0.5 ppm, you have to use:

  • Calcium hypochlorite 67%: Use 0.10 oz for every 1000 gal of pool water.
  • Calcium hypochlorite 75%: Use 0.02 oz for every 1000 gal of pool water.
  • Lithium hypochlorite 35%: Use 0,19 oz for every 1000 gal of pool water.
  • Sodium hypochlorite 12%: Use 0,54 fl oz for every 1000 gal of pool water.
  • Chlorine gas: Use 0,07 oz for every 1000 gal of pool water.

If you like to raise the chlorine level from 0 to 2 ppm in a 10,000 gal pool, you need 3.50 oz of calcium hypochlorite (75%).

You now know that you need to calculate the exact pool volume to optimize the chlorine content with this information.

Tip: You must keep an eye on your chlorine content during the first 2 weeks of initial filling because there can be strong fluctuations. You can read more about pool maintenance for above-ground pools in my Intex Pool Guide.

How often to add chlorine to a pool?

You need chlorine in your pool to keep the pool water clean. But how often should you add chlorine?

It happens that beginners who do not know the answer to this question add chlorine all the time.

You need to add chlorine every day if:

  • The pool is used by a lot of people.
  • The water in the pool is very dirty (for example after rain).
  • There are other large pollution sources in or near your home.

Generally, you need to add chlorine only once every 3-4 days.

When the chlorine level in the pool is too low, even after adding it according to your routine, repeat the addition until the appropriate value is reached.

In most cases, this means that if the chlorine level drops from 2 to 0.5 ppm over a day or two, you need to add it once more.

In no case do not wait for more than 48 hours since the last chlorine test – The water in the pool would, without chlorine, not stay clean for long in the summer.

A chlorine tablet in the floater can be handy.

How many chlorine tablets to add to the pool per week?

Chlorine Tablets are handy to produce free chlorine in your pool.

They usually dissolve within 24 hours (although some will last for several days).

With the help of a floater or a dosing system, chlorine is continuously added to the pool water.

This way, you don’t have to worry so much that there is not enough chlorine in the pool.

But how many chlorine tablets do you have to put in the floater per week?

It’s hard to say because it’s difficult to dose the chlorine with a floater.

I recommend putting 1 chlorine tablet in the floater and testing the chlorine level. If there is too little chlorine in the water, you can simply add another chlorine tablet.

Normally, 1 chlorine tablet per week is enough to keep the pool clean with free chlorine.

Tip: Hang the floater in the pool’s return to better distribute the chlorine.

What can I use instead of chlorine tablets?

If, for whatever reason, you currently do not have chlorine tablets, then as an alternative, you can use:

  • Chlorine granules
  • Liquid chlorine

It is important to note that the strength of these chemicals differs, and you should choose accordingly.

The difference to chlorine tablets is that tablets produce slowly free chlorine and chlorine granules contains less free chlorine.

How to increase free chlorine in the pool fast?

If your chlorine level drops below 1.0 ppm, add a chlorine product to bring it back up to around 1.5 to 2 ppm.

If you need to quickly raise the free chlorine level in the pool (such as when there is an ongoing algae bloom), use liquid chlorine bleach containing sodium hypochlorite.

Chlorine granules will take a bit longer to raise the chlorine level, but they are just fine for most purposes. However, one downside is that chlorine granules won’t melt down algae as effectively as liquid bleach (but this isn’t usually an issue).

I also recommend using a chlorine tablet in the skimmer or floater to increase the free chlorine level in your pool fast.

How much chlorine do I need to shock my pool?

Pool shock contains a lot of free chlorine that can destroy all contaminants in the pool within a few hours.

If you shock your pool with chlorine, there is normally no need to add more chlorine for at least 24 hours. This timeframe varies from pool to pool, so check again after 24 hours, and if the water isn’t clear, add more.

But how much do you need to add?

There is no fixed amount of chlorine to use for shocking your pool. It really depends on the pool water volume.

You need to completely shock your pool with 3 to 5 ppm free chlorine added over 12 hours.

  • If you have a pool of about 15,000 gallons, and your current chlorine level is about 1.5 ppm, then add around 10.50 oz of calcium hypochlorite (67% chlorine).
  • For 20,000 gallons, you will need to shock your pool with 14.00 oz over the same period.

The next day, test your chlorine values. When the chlorine level has fallen below 2ppm, swimming in the pool is allowed again.

Can you shock a pool without the pump running?

The water must circulate when you shock your pool to bring down the chlorine level. Generally, you will use a pool pump for this.

However, if you do not have a pool pump or broken down, you can also shock the pool without a pump.

But you can also shock your pool without a pump running.

For this purpose, chlorine granules are scattered over a large area on the ground with a small shovel in still water. The pool is left to rest for 12 to 24 hours so that a chlorine cushion builds up.

After that, however, the water must be circulated to reduce the chlorine level to a normal level.

How to lower the chlorine level in your pool?

A high chlorine level is not as dramatic as a high pH level, but it can still cause problems if not corrected.

Too much chlorine in your pool water is not good for the swimmer’s eyes and skin.

To lower chlorine levels in your pool, follow these simple steps:

  1. Start your pool pump to lower the chlorine level naturally.
  2. Test the free available chlorine (available chlorine) and total chlorine (combined chlorine). You get this information by using pool test strips or reagent testing equipment. If using pool test strips, look for the darkest blue or purple zone color. Using pH reagent testing equipment, this is referred to as chlorine px.
  3. To lower chlorine levels in your pool, add water clarifiers that contain polymers (products containing cyanuric acid are examples of clarifiers).

If you like to lower the chlorine level fast, try to drain 1/5 of the pool water (a partial drain).

This will immediately lower the chlorine level. But afterward, you have to balance the pH and alkalinity level in your swimming pool.

Tip: There are also special chlorine reducers as granules. But with the steps mentioned above and a little time, this agent is not always necessary.

Why is the pool green, but the chlorine is high?

A common problem for beginners is that the chlorine level is high, and the pool is green.

Why is that? Chlorine is supposed to kill the algae in the pool?

Several things could cause this:

  • If you add too little chlorine, then it can’t kill all of the algae, and there will be a ‘reserve’ left in the water, which will grow back fast. So always make sure to add enough chlorine to your pool (1 to 2 ppm is okay).
  • Too much sun will make your chlorine evaporate faster. This is especially the case with unstabilized chlorine. To prevent this, you must use cyanuric acid.
  • Another reason why the pool is green – despite sufficient chlorine levels – may be the cyanuric acid. Cyanuric acid should not exceed 50 ppm in the pool water because this reduces the effect of chlorine.
  • It can also be that the pH value in the pool is too high, and the chlorine does not develop its effect optimally. Make sure that the pH value is between 7.2 and 7.6.

And then, there are also cases where the filter system cannot filter all of the water. If there’s too much dirt in your pool, it takes more chlorine to purify that amount of water.

Do you have further questions about chlorine?

The best way to contact me is @contactswimfool on Twitter or Pinterest. I am happy to help you.

Have fun swimming!

Photo of author
Hi. I'm Max Berg. I've been in the pool industry since 2015 and have always felt drawn to water. I'm the author behind swimfool.com, where I share my years of experience in pool maintenance and give helpful tips on keeping a swimming pool or hot tub clean. My tips reduce the costs of water treatment and protect the environment.

Keep Reading