Pool Green But Chlorine High

My Pool Is Green But Chlorine Is High - Why? How Do I Fix It?

If your pool is green even though your chlorine levels are high, it can be confusing.

Green colour being present in your pool typically means algae is growing, but many pool owners believe that high chlorine levels should fully prevent this. In reality, there are many reasons that your pool water may still be green even though your chlorine levels are high.

In this article, we will discuss the causes of a pool being green even though chlorine levels are high, how to diagnose the specific cause of your green water, and how to fix this problem with your pool.

Causes of Green Pool With High Chlorine

Pools with water that is green in colour yet are maintaining high chlorine readings often have other problems.

These may include other chemical imbalances that reduce the effectiveness of your chlorine, problems with your pump or filter, or a large concentration of small particles of organic debris.

In order to determine the exact cause of your green pool, you may need to perform many tests.

It’s important to remember that chemical imbalances cannot be diagnosed by colour alone, so attempting to treat your pool for these conditions without properly testing the chemistry will yield undesirable results.

High Cyanuric Acid (CYA)

In terms of chemical imbalances that can cause green pool water with high chlorine, the most common culprit is high cyanuric acid (CYA).

While some CYA is necessary to ensure chlorine is effective over a wide pH range, and in outdoor pools to prevent chlorine from being depleted by the sun, excess CYA does more harm than good. It reduces the amount of hypochlorous acid (HOCl) in solution, which is the sanitising form of chlorine.

Excessive cyanuric acid levels can be caused by using an abundance of a stabilised chlorine product as a shock treatment when an unstabilised product should have been used instead.

You can test the CYA content of your pool using a test kit. Ideal levels for CYA in a pool range from 30-50 parts per million (ppm). If your pool tests higher than 50ppm, then the cause of your green water is most likely a chlorine ineffectiveness problem due to an excess of stabilisers.

It is recommended that you maintain a free chlorine level of at least 7.5% of the cyanuric acid level. For example, if you have CYA of 100 ppm, you need a free chlorine level of 7.5 ppm for it to be effective.

This is why your pool might be green even if the free chlorine reading is 5 ppm.

Imbalanced pH Levels

High pH levels can also cause your pool water to remain green even with high chlorine levels.

This is especially true in pools that don’t use stabiliser, such as indoor pools, or poorly maintained outdoor pools.

This is because unstabilised chlorine rapidly becomes ineffective once the pH exceeds 7.6. Most of the hypochlorous acid ends up dissociating and forming the hypochlorite ion (OCl).

High Phosphates

Although less likely than the previous two causes, another potential cause of persistently green pool water even with high chlorine levels is a high concentration of phosphates in your water.

Phosphates are an organic, natural nutrient that is released from decomposing organic debris in your pool, such as leaves or stray lawn trimmings.

Phosphates themselves act as a perfect food source for algae, allowing them to flourish in large numbers, potentially even with chlorine present.

Test your phosphate levels with a test kit. Pools with phosphate levels above 500 parts per billion (ppb) are at an increased risk of algae growth and require a removal treatment.

Recent Rainstorm

If it has recently rained, the influx of freshwater along with organic debris may have reduced your chlorine levels more than you think. Check out this guide on dealing with a green pool after rain.

Pump or Filter Problems

One final cause of green pool water that persists even though you have high chlorine levels is issues with your pool’s pump or filter system.

If this system is clogged, needs cleaning, or is not being run for an adequate number of hours each day, your pool is likely to develop algae along with other unsanitary water conditions.

We recommend running your pool’s pump and filter unit for an absolute minimum of 8 hours per day in order to avoid debris accumulation and bacterial blooms.

Finding the Right Treatment

Lowering Cyanuric Acid

Unfortunately, there is no chemical remedy to remove high concentrations of cyanuric acid from your pool. Instead, CYA must be removed from your water by performing a large water change.

To do this, simply drain a large portion of your pool’s water and refill it with fresh, source water. It is difficult to estimate the amount of water you will need to replace, but higher concentrations of CYA will necessitate that larger amounts be removed.

Don’t remove too much water from your pool at once, as you risk the pool ‘popping’ out of the ground, particularly in areas with a low water table.

Balancing Your pH and Total Alkalinity Levels

Your pool should ideally have pH levels between 7.2-7.6 and total alkalinity levels that range from 80-120 ppm. If these levels are reading higher or lower than these ideal ranges, you will need to make adjustments.

If your pH and total alkalinity levels are too high, you will want to use a pH decreaser. This product contains sodium bisulphate (a dry acid) that lowers your overall pH and alkalinity.

If your pH levels are too low, you will need to use a pH increaser, which typically contains soda ash.

If pH is fine but you want to raise alkalinity, consider using an alkalinity increaser, which typically contains sodium bicarbonate (baking soda).

Use a Phosphate Remover

If your pool reads at or above 500 ppb of phosphates, you will need to use a phosphate remover. These products are typically referred to as phosphate starvers, and occasionally have other side effects.

We recommend Lo-Chlor Starver M, because unlike other comparable products, this formula is not known to cause temporary pool cloudiness during use. If you have especially high levels of phosphates, you may need a more concentrated product, such as LoChlor Starver X.

Clean or Replace Your Filter

Filters that are not routinely cleaned can harbour a wider variety of algae, bacteria, and other debris. If you have a traditional filter system with a removable cartridge, it is important to remove and thoroughly rinse this cartridge on a monthly basis, or after any instances that considerably lower your water quality.

To clean your filter, scrub it with a soft bristled brush and rinse it with clean water, using a spray nozzle if you have one available. If you cannot get your filter reasonably clean, or if it shows signs of damage, you may need to replace it.


Pool water that stays green even when you have high chlorine levels can be confusing. Luckily, once you identify the source of this green pool water, you can correct the cause of it and restore your pool to its former clarity.

Do you have any questions about having green water even while chlorine levels are high? We’d love to hear from you in the comment section!

Louis from Pool Advisor


A chemical engineer by trade, Louis is committed to debunking myths in the pool industry by explaining the underlying chemistry and making it accessible to all.