7 Reasons Your Pool Won’t Hold Chlorine (And How to Fix It)



Are you continuously adding chlorine to your pool but it’s not doing anything?

This article will walk you through all the likely reasons your pool won’t hold chlorine, and how to fix each potential problem.

What Does “Hold Chlorine” Even Mean?

Well, there are two ways to interpret this:

The first is if you’ve tested your water soon after adding chlorine only to find you’re somehow seeing no free chlorine in your pool.

The second is if your water always seems to look dirty or green despite adding plenty of chlorine and getting a normal free chlorine reading.

Either of these scenarios is bad news for you as a pool owner, but they do each offer a clue as to what might be causing your pool water to ignore chlorine, and how you might go about fixing it.

Why Your Free Chlorine Reading Is Key

If your chlorine isn’t working effectively, one of the easiest ways to narrow down the cause of the problem is to test your water for free chlorine.

This result will tell you a LOT about what’s happening to the chlorine in your water.

Specifically, if you’re seeing no free chlorine in your pool, even after recently adding chlorine to your water, it means your chlorine is either burning off or being consumed very quickly.

Alternatively, if you’re seeing a free chlorine reading above zero despite having less-than-ideal water conditions, that can only point to your chlorine being inhibited by the conditions of your water.

We’ll talk about possible causes and fixes in the next sections, but it’s important to first run this test so you have a general idea of what’s going on.

Note: When troubleshooting issues with your pool chemistry, we strongly recommend using a liquid drop kit for more accurate testing.

Why Your Pool Won’t Hold Chlorine (Meaning You Have No Free Chlorine)

If you can’t get your free chlorine level up despite adding more chlorine to your pool, either your chlorine is being quickly burned off or it’s being consumed by some serious contamination.

Let’s run through the most likely causes:

1. You Have No Stabilizer

As effective as chlorine is at sanitizing pools, it has one major weakness.

When exposed to the sun’s UV rays, even in water, chlorine is literally burned off and released into the air.

This is why outdoor pools, or any pool under direct sunlight, require a chlorine stabilizer (cyanuric acid) to shield their chlorine from the sun. Without a stabilizer, free chlorine can disappear from an exposed pool in a matter of minutes.

This is also why shock treatment is often added to a pool at night, as it ensures it has enough time to work before the sun comes into play.

Solution: Add 30 to 50 parts per million of cyanuric acid to your pool in order to protect it against UV light.

2. You Have an Algae Bloom

Algae is nasty stuff; once it begins to take hold, it can take a huge amount of sanitizer to properly kill it off.

This is what’s known as having a ‘high chlorine demand’, and it means your daily chlorine level gets used up almost instantly as it tries to fight off a potent source of contamination.

While even early-stage algae are often visible, it’s not always obvious you have algae just by looking at your water.

Solution: Brush your walls to dislodge algae and shock your pool to completely kill everything in the water. This requires a chlorine dose that’s 10 times stronger than your usual level.

3. You Have Other Contaminants

If your pool isn’t contaminated by algae (by far the most likely cause), the culprit may be a little harder to identify.

For example, it could be any combination of the below:

  • Rains or storms, which may add pollutants to your water.
  • Bugs, including water bugs, worms, or mosquito larvae.
  • Biofilm; a slimy layer of bacteria that forms in your equipment.
  • “Little accidents”, especially if young children use the pool.
  • Bather waste, particularly if you recently threw a pool party.
  • Pets using or drinking from the pool, such as cats or dogs.
  • Wild animal waste in the pool (ducks, frogs, etc.)

Either way, these contaminants can create an equally high chlorine demand, leaving you with little to no reserve for your regular, daily sanitation.

Solution: This follows a similar process to when you have algae in your pool; simply scrub the walls, shock the water, and run the filtration system until your pool water appears clear again.

Why Your Chlorine Isn’t Working (But You Still Have Free Chlorine)

If you can maintain a free chlorine reading but your water still looks dirty, it means your chlorine is being inhibited by something in the chemistry of your pool water.

Again, let’s cover the most common causes:

4. Your Stabilizer Level is Too High

While having no stabilizer can lead to losing all your chlorine, having too much stabilizer can render your chlorine over-stabilized (or ineffective).

This is often referred to in the pool industry as ‘chlorine lock’.

Put simply, cyanuric acid “locks up” a percentage of your chlorine to protect it from sunlight. When your stabilizer level is too high, however, it can consume as much as 100% of your chlorine which prevents it from effectively sanitizing your water.

Like most things in pool chemistry, it’s all about balance.

Solution: Keep your cyanuric acid level ideally between 30 and 50 parts per million (ppm), and certainly no higher than 100 parts per million. You may need to partially drain and refill your pool.

5. Your pH Level Is Too High

Chlorine works best at lower pH levels.

In fact, chlorine is incredibly potent in acidic water, but this would be a far lower pH level than you’d want to see in a pool.

A pH level of 7.2 to 7.8 offers the best balance between chlorine effectiveness and swimmer comfort. Going over this pH limit inhibits chlorine’s sanitizing power too much, and leaves your water under-sanitized.

Unfortunately, pH will often rise over time, especially in a salt pool.

You can check your pH level with a test strip or liquid test kit, and any result over 7.8 should be acted upon.

Solution: Use muriatic acid to bring your pH level down into the appropriate range. Be aware, though, you may also need to follow acid with baking soda to rebalance your total alkalinity.

6. Your Phosphate Level Is Too High

Phosphates (and nitrates, for that matter) are algae food, and these can be found in swimming pools to varying degrees.

If you have a particularly high phosphate level, this can make algae even more aggressive and force your chlorine to work harder to keep it at bay. This will indirectly use up a little more of your free chlorine, leaving less of it in your water to deal with everyday contaminants.

This gives the impression your chlorine isn’t working when it’s actually working overtime.

With that being said, phosphates aren’t nearly as problematic as some pool experts will have you believe. After all, algae won’t even be able to get going if you just maintain a high enough free chlorine level.

Solution: Maintain a free chlorine level above 3 parts per million and you won’t have to worry about algae even forming, let alone feeding.

7. Your Water Isn’t Circulated Well Enough

Circulation is everything when it comes to maintaining a healthy pool.

Without adequate circulation, chlorine is unable to properly penetrate your water which leaves your pool under-sanitized in places.

If that wasn’t bad enough, testing an under-sanitized area of your pool will give you a false negative when taking a free chlorine reading, leading you to believe there’s no chlorine in your pool when, in fact, there is.

Solution: Ensure your pump is running long enough throughout the day to completely cycle your water, and clean/replace your filters to ensure your filtration system is running efficiently.

Do You Have a Saltwater Pool?

If you have a salt pool, it’s also possible your saltwater generator is having trouble making or maintaining chlorine.

There are a number of reasons for this, including:

  • Not running the generator long enough.
  • Not having enough salt in the pool.
  • Strong calcium buildup in your salt cell.
  • A fault with the generator (blown fuse, broken interface, etc.)
  • Not enough stabilizer (cyanuric acid)

Most generators will let you know of any problems through an indicator light, so always pay attention to what your interface says.

You should know: Free chlorine naturally sits lower in a saltwater pool because it doesn’t rely on large dumps of chlorine, so don’t expect the same chlorine reading as you’d see in a traditional chlorine pool.

Can’t Keep Your Chlorine Up?

Don’t worry, it happens to the best of us.

Identifying the root of the problem is all about testing your water and narrowing it down through a process of elimination. Once you have that, the fix is usually the easy part.

Categories: Pool Care, Pool Chemistry