Stabilized vs Unstabilized Chlorine (Here’s the Truth)



When it comes to sanitizing a pool, you first need to decide between using stabilized or unstabilized chlorine.

In this article, we’ll go over the key differences between them, why these differences matter, and which one should you use in your pool.

Let’s get into it.

Quick answer: Stabilized chlorine adds cyanuric acid which eventually becomes too high and makes your chlorine ineffective. For most pools, using unstabilized chlorine and cyanuric acid individually (as needed) is best.

What is Stabilized Chlorine?

Stabilized chlorine is made with stabilizer, also known as cyanuric acid.

Cyanuric acid, (often abbreviated to CYA) protects chlorine from the sun’s UV rays, preventing it from breaking down too quickly — especially in outdoor pools that are often exposed to direct sunlight.

It works by binding to the chlorine molecules, forming a protective layer around them while still allowing the chlorine to sanitize the pool effectively.

This chlorine is primarily used as a residual sanitizer—meaning it’s added to the pool frequently in order to maintain a stable chlorine level—but it’s also occasionally used in shock treatments.

There are 2 types of stabilized chlorine:

1. Trichlor

Trichlor, or trichloro-s-triazinetrione, is the most popular chlorine type of stabilized chlorine for residential swimming pools.

  • Most chlorine tablets are trichlor
  • It’s sometimes found in granular or powder form
  • Granules and powder are usually first dissolved in a bucket
  • It’s rarely used as shock treatment (but can be)
  • It contains 50-58% cyanuric acid by weight
  • It has 90% available chlorine by volume (chlorine efficiency)

Note: When talking about “available” chlorine, we’re referring to how much of the chlorine (by weight) is actually used to sanitize the water. We like to use the phrase “chlorine efficiency” to describe this.

2. Dichlor

Dichlor, or sodium dichloro-s-triazinetrione, is another type of stabilized chlorine, often used for both regular chlorination and shock treatments.

  • It dissolves faster than trichlor
  • It’s often found in granular or powder form
  • It’s usually first dissolved in a bucket of water
  • It’s less commonly found in tablet form (but can be)
  • It’s commonly used as a shock treatment in small pools and spas
  • It also contains 50-58% cyanuric acid by weight
  • It has 56-62% available chlorine by volume (chlorine efficiency)

Note: Despite dichlor being “weaker” than trichlor, the fast-dissolving nature is better suited to some applications, especially for shocking.

What is Unstabilized Chlorine?

Unstabilized chlorine is NOT made with chlorine stabilizer (cyanuric acid).

As a result, this chlorine doesn’t offer any protection against UV light and will degrade much faster when exposed to direct sunlight — though this often isn’t an issue for indoor or well-shaded outdoor pools.

Chlorine in solid or liquid form must be bound to a substance to make it easier to handle, so unstabilized chlorine is made with substances like calcium and sodium instead of cyanuric acid.

It can be used as a residual sanitizer or a shock treatment, and it can also be paired with cyanuric acid later on

There are 3 types of unstabilized chlorine:

1. Cal-Hypo

Cal-hypo, or calcium hypochlorite, is the most popular type of unstabilized chlorine when it comes to shock treatments.

  • Most commonly found in granular or powder form
  • It’s sometimes found in tablet or puck form
  • It’s often used as a shock treatment (fast-dissolving)
  • It increases your calcium hardness level
  • It has around a 65-75% chlorine efficiency

2. Liquid Chlorine

Liquid chlorine, or sodium hypochlorite, is widely considered the best unstabilized type of chlorine, especially for all-purpose use.

  • It comes in a jug of 85-90% water (pre-diluted)
  • It’s used as both a residual sanitizer and shock treatment
  • It adds sodium to your water (increases salinity)
  • It usually comes in concentrations of 10-15%.

Note: While liquid chlorine is very weak relative to other types, it’s also used in much higher doses to make up for the deficiency.

3. Lithium-Hypo

Lithium-hypo, or lithium hypochlorite, is harder to come by and by far the least popular type of unstabilized chlorine.

  • Commonly found in granular form
  • Dissolves relatively quickly compared to other forms
  • Has around 35% available chlorine

Stabilized vs Unstabilized Chlorine

Now that you know the different characteristics of stabilized and unstabilized chlorine, let’s tackle the limitations of each type.

Here’s where things get tricky:

The Problem with Stabilized Chlorine

If your pool is exposed to direct sunlight throughout the day, you need cyanuric acid to stabilize and preserve your chlorine.

Without CYA to shield your chlorine from UV, you can lose up to 90% of your free chlorine level within a few hours of topping up — making it financially and practically unsustainable.

Stabilized chlorine is an attractive solution because it combines chlorine and cyanuric acid into one convenient product, but there’s a catch.

Chlorine gets used up over time while cyanuric acid doesn’t; it stays in the water and continues to accumulate every time you add more stabilized chlorine until it eventually “locks up” your chlorine.

In this locked-up state, too much of your chlorine is bound to cyanuric acid and unable to effectively sanitize, leaving your pool vulnerable to all kinds of contaminants, including bacteria, viruses, and algae.

While this can take time to manifest, you will notice that more chlorine is required over time to maintain the same sanitizing power, until it eventually becomes unsustainable.

What’s more, the only way to fix this problem is to partially drain and refill your pool, which can be both expensive and time-consuming.

The Problem with Unstabilized Chlorine

The same rule applies here; if your pool gets any sunlight, you will almost certainly need to stabilize your chlorine.

So where does that leave unstabilized chlorine?

Well, on the surface, unstabilized chlorine is only suitable for pools that aren’t exposed to natural light, which is largely exclusive to indoor pools (and not even all indoor pools, for that matter).

If you don’t have to worry about sunlight eating up your chlorine, there’s no reason to add cyanuric acid to your water, which makes unstabilized chlorine sources like cal-hypo and liquid chlorine the ideal choice.

But if you do have to worry about sunlight, unstabilized chlorine simply isn’t going to work for you — at least by itself, anyway.

So Which One Should You Use?

On one hand, using stabilized chlorine protects and preserves your sanitizer level while slowly crippling it over time.

On the other hand, using unstabilized chlorine will cause your chlorine to break down too quickly when exposed to sunlight.

Fortunately, there is a middle ground; a “best of both worlds”, if you like — and it’s an approach we recommend to most pool owners:

The trick is to use unstabilized chlorine and add cyanuric acid separately.

Here’s why this works so well:

  • Using unstabilized chlorine allows you to add chlorine without accumulating too much stabilizer over time.
  • Using cyanuric acid separately allows you to use it only as needed and effectively maintain a suitable cyanuric acid level.

Yes, it requires a little more work to manage, but it saves you a lot more work in the long run because you won’t have to deal with sanitizer issues that stem from having a high cyanuric acid level (chlorine lock).

Note: Cal-hypo adds calcium to your water which will increase your calcium hardness and potentially lead to calcium scaling. For this reason, liquid chlorine is often considered a better source of unstabilized chlorine.

The Bottom Line

Overall, stabilized chlorine products—like chlorine tablets—provide convenience today but at tomorrow’s expense.

Instead, coupling unstabilized chlorine—like liquid chlorine—with cyanuric acid gives you granular control over both levels, and it adds only a minor inconvenience to your pool maintenance routine.

Categories: Pool Care, Pool Chemistry