7 Reasons Why Pool is Green but Chlorine is High (And How to Fix)

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A sparkling blue pool is one of the best ways to cool off on a hot summer day.  However, waking up to a pool that has turned green overnight can quickly damper any excitement.

One of the first actions taken by many pool owners is to check the water balance.  Oftentimes if chlorine has been depleted, it provides an easy path for an algae bloom to take over.

However, there are actually times when the pool is green but chlorine is high.  This often befuddles even the most experienced pool owners, as high chlorine is often thought to prevent green water.

In this post, we’ll detail several reasons why the pool is green even though the chlorine is high.  We’ll also look at ways to prevent this scenario from occurring in your pool.

Good vs Bad Chlorine

Before we review why a pool turns green even if the chlorine is high, it’s important to define the types of chlorine measurements.  Chlorine in the pool can be grouped into three different categories, which vastly differ from each other.

Free Chlorine

Free chlorine (FC) is the most beneficial type of chlorine in the pool.  FC indicates the amount of chlorine that is available to fight off organic contaminants, including bacteria and algae.  

The desirable amount of FC in a pool is often 2 to 4 ppm, but highly depends on the amount of cyanuric acid (CYA) in the water.

Free chlorine is consumed as it combines with organic contaminants to kill them off.  It is also broken down through the sun’s UV rays.

High quality pool test kits can test for free chlorine level using a FAS-DPD test.  This will measure FC to tell you exactly how much is available in the water to prevent any bursts of harmful growth.

Combined Chlorine

Combined chlorine (CC) is the amount of chlorine that has reacted with organic compounds in the water.  It is a less desirable type of chlorine since it is no longer in an available form to fight off any additional contaminants.

If the pool has high levels of CC then it could indicate high amounts of contaminants that have been eating up FC, leaving the pool more vulnerable to unwanted growth.  

CC includes chloramines which indicate reaction with nitrogen compounds, such as urine and sweat.  

The chloramines in CC also provide that well-characterized “chlorine” odor often associated with pools.  Contrary to popular belief, that “chlorine” smell is not generally a good sign as it indicates high amounts of organic compounds.

Similar to free chlorine, combined chlorine can also be measured using the FAS-DPD test.  A CC concentration of more than 0.5 ppm often indicates a problem, and shocking the pool may be necessary.

Total Chlorine

As the name implies, Total Chlorine (TC) represents the sum of Free Chlorine and Combined Chlorine.  

If the total chlorine is higher than the free chlorine in the water, then this indicates presence of the combined chlorine.  This doesn’t necessarily indicate a problem, unless the combined chlorine is higher than the free chlorine.

What is More Important Free Chlorine or Total Chlorine

While it’s pretty evident that combined chlorine is not necessarily beneficial to the pool, some may wonder whether free chlorine or total chlorine is more important.

Most basic pool test kits will provide measurement of the total chlorine.  The total chlorine tests often use yellow OTO dye or the quick test strips.

Unfortunately, these basic test kits won’t indicate how much free chlorine or combined chlorine is in the water.  In order for the total chlorine measurements to be useful the water needs to be “clean”, in which case the total chlorine represents the free chlorine as well.

Since it’s often difficult to visually quantify “clean” water, the basic total chlorine tests only go so far in representing free chlorine.  Ultimately, it’s recommended that a higher-end test kit with FAS-DPD is used in order to truly determine how much of the good free chlorine is present.

A high total chlorine reading could provide false confidence that there is adequate chlorine in the water. In reality, a high total chlorine reading could still signify high levels of combined chlorine with inadequate free chlorine.

Since free chlorine is what actually keeps the pool water sanitary, it’s usually more important than total chlorine. However, total chlorine readings still have their place in pool chemistry. Total chlorine in conjunction with free chlorine can be used to determine the combined chlorine level, which can signify how much chlorine is being consumed.

Why Pool is Green but Chlorine is High

Walking out to a green pool can be frustrating.  But, frustration can quickly turn to confusion when you measure the water chemistry to find that chlorine is actually high!

Fortunately, there are often some simple causes that can be identified for the pool being green while the chlorine is high.

Green Algae Growth

As we reviewed earlier, free chlorine is required to combat organic growth such as algae.  

If an algae bloom has formed while the chlorine level is high, it’s possible that the combined chlorine is higher than the free chlorine.  This indicates that the pool likely doesn’t have enough free chlorine to counteract the algae bloom.

In such situations, you should consider shocking the pool to really elevate the free chlorine level.  When shocking the pool, you should aim to reach a free chlorine level that is at least 10 times that of the combined chlorine.

You can use an FAS-DPD test to determine the actual amount of free chlorine and combined chlorine in the pool.  With this information you can better calculate just how much shock should be added to reach the necessary free chlorine level. 

Using an algaecide can also help rid the pool of algae.  Even if using algaecide, you’ll still want to ensure adequate free chlorine level to keep the pool sanitary.

Mustard Algae Growth

Mustard algae can vary in appearance but can sometimes include a yellowish-green appearance.  

This type of algae is closely related to green algae but is resistant to chlorine.  So, even shocking the pool may not be enough to permanently get rid of mustard algae, as it will tend to resurface.

Using an algaecide will help kill off mustard algae. 

Additionally, raising the FC level 50% higher than normal shock level should also be enough to get rid of this pesky growth.  For instance, normal shocking without presence of mustard algae requires FC of 16 ppm when CYA is 40. However, when battling mustard algae you may need to increase the FC to 24 ppm when shocking at the same CYA.

pH is High

pH can have a significant impact on the effectiveness of free chlorine in the pool.  If the pH is high, the effectiveness of chlorine is reduced.

A target pH of 7.2 to 7.6 is often recommended in pools.  At this ideal range, the effectiveness of chlorine is about 50%.  In comparison, if pH is 8.0 the chlorine effectiveness is cut down to 25%. 

A pH of 8.0 means the pool requires double the amount of chlorine to maintain the same effectiveness as a pool with pH of 7.5.  So, a high chlorine reading with pH of 8.0 may not actually be high enough to fend off unwanted growth such as algae.

This emphasizes the importance of maintaining appropriate water balance beyond just chlorine.  pH can be reduced using muriatic acid and can be increased using baking soda.

Imbalanced pH can also cause damage to pool components, such as increasing the risk of rust on metal coping.

Poor Circulation

If the pool’s circulation is subpar, it can affect the distribution of chlorine in the water.  Instead of chlorine levels being evenly distributed throughout the water, there could be areas of high chlorine and areas of low chlorine.

Those low chlorine areas could become a breeding ground for algae that could overtake the entire pool.

Ensuring the return jets are pointing in the right direction will promote full circulation of the water.  Often, angling the jets slightly downward and in the same direction can help ensure all areas in the pool are receiving chlorinated water.

Pump Not Running Long Enough

If the pump is not running long enough, it may prevent adequate filtration of the water.  This means a portion of the water is not being filtered or treated daily which could allow algae to form.

Many pool owners assume that running the pump 8 hours a day is enough.  While this may be true for some pool systems, it’s by no means adequate for all pools.

You should aim to run the pump long enough to provide a complete turnover of water in the pool each day.  For some pools this may be 8 hours and for other pools this may be 12 hours.  

There are various means to determine how long to run the pump each day, including simply using a pool noodle and stopwatch.

High Levels of Metal

Certain metals in the water, including iron and copper, will oxidize when exposed to chlorine.  If the level of these metals is high enough they can actually cause the water to turn green when reacting with chlorine.

The combination of these metals and chlorine can actually cause blonde hair to turn green.

Iron is often introduced through the water supply used to fill the pool.  While municipal water may contain some iron, it is more prominent in well-water systems.

Copper is often introduced through pool equipment, such as heaters.  It can also be introduced through certain algaecides that contain copper, since it can help fend off algae.  It’s somewhat ironic that copper itself can actually cause green water, since it’s often used to keep algae from turning the pool green.

If the pool has high levels of metal, a sequestering agent can be used to bind to the metals and prevent them from turning the pool green.

Too Much Stabilizer

Cyanuric acid (CYA), also known as stabilizer, is an important aspect of pool chemistry.  An ideal CYA level in a pool ranges from 40 to 70 ppm, but can vary depending on the type of chlorination used.  

Stabilizer helps protect chlorine from breaking down quickly under the sun’s UV rays.  However, too much CYA can reduce chlorine’s effectiveness.  

If the pool has too much stabilizer, even high levels of chlorine may not be enough to keep the pool from turning green due to algae.  For instance, a CYA of 40 requires a minimum FC of 3, whereas a CYA of 90 requires a minimum FC of 7.

To correct a high level of CYA, it’s recommended that a partial drain and refill is performed.  This will dilute the CYA in the water helping lower it to the desired level.  

Alternatively, you can try to live with high CYA levels, but will need to also maintain an even higher amount of free chlorine in order to keep the water clear.

Is It Safe to Swim in Green Pool Water?

Pool day plans can come to a screeching halt if the water has taken on a green appearance.  While a green pool does not always mean swimming should be avoided, it’s important to fully assess the situation.

Green water due to algae can pose potential health risks as it often means the water is not sanitized.  Although algae itself doesn’t typically pose much risk, its presence could mean that other harmful bacteria are running rampant.

Before giving the green light to swimming, you’ll want to measure the pool chemistry.  This includes free chlorine, pH, and stabilizer levels as these will help provide indication as to how effective the chlorine is.

Green water due to metals is less of a health concern, but could cause potential staining of hair or swimwear.

Ultimately, if the water is so green that you cannot see the bottom then this can pose an increased risk of drowning.  A swimmer could sink to the bottom without others being able to see that someone is down there.  In this case, it’s best to allow time for the water to clear up through proper chemical treatments and increased filtration.

Why Does My Pool Turn Green When I Add Chlorine

Another situation that can complicate pool care is when the pool turns green shortly after adding chlorine.  In some cases, you may actually have just shocked the pool when it was crystal clear, only to find that it turned green a few hours later.

In such cases, this may indicate the presence of high levels of copper or iron.  These metals will react with chlorine and can turn the water green.  

You can confirm the presence of metals by using an appropriate test kit for copper or iron.  

To reduce the metals you can add a sequestering agent to the pool to keep them from reacting to chlorine.  This can help keep the pool from turning green when adding chlorine.

Final Takeaway

While a green pool with high chlorine levels can be confusing at first, there are several simple causes.

First and foremost, the water chemistry should be checked to ensure adequate free chlorine readings in conjunction with the CYA level. Lack of free chlorine could lead to algae growth which is one of the most common reasons for a green pool.

High metal content in the pool can also change water from blue to green in the presence of chlorine. And, ensuring the pump runs long enough each day can further prevent the green pool from cropping up due to poor circulation.

Once you’ve determined the root cause of the green pool, you can then take the appropriate steps to get it back to its crystal-clear glory!

Happy Swimming!

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