When to Shock your Pool

pool chlorine shock, dichlor or cal hypo granular pool chlorineMany pool owners find that shocking is almost always required to get rid of algae when opening your pool in the spring, but do you know when it is necessary at other times of the year?

It’s certainly true that regular shocking kills bacteria and algae that may be slowly building up in your pool, but the main reason to shock your pool on your regular basis is to remove the buildup of chloramines.

Have you ever been in a pool and noticed the intense smell of chlorine and had skin or eye irritation?  It would be logical to assume that this means there is simply too much sanitizer in the water, but it actually is quite the opposite. This smell is indicating that there is a buildup of chloramines (or combined chlorine) in the water.

Combined chlorine, or chloramines – develop whenever free chlorine combines with nitrogen or ammonia in the water. Nitrogen and ammonia are generally introduced to the water via swimmer waste (sweat, saliva, urine, etc.). It can also come from fertilizers or mulches blowing or washing into the pool.

After they combine into a chloramine, they are essentially “used-up” chlorine and do not provide the same sanitizing benefit as free, available chlorine. In fact, chloramines are about 80 times less effective at killing bacteria than free chlorine.

How to Know When to Shock a Pool

how to know when to shock a poolA strong chlorine smell, or red burning eyes are indications that your chlorine that is present in the water is less able to kill bacteria and algae. But there are more scientific methods.

Weekly shocking will prevent the buildup of chloramines in the pool, but the best way to know if you need to shock is to use a test kit to determine your free and total chlorine levels. As you may have guessed, “total chlorine” is the level of free and combined chlorine in the water.

The difference between free and total chlorine is the level of combined chlorine. Whenever that level is 0.3 ppm or higher, you should shock the pool to reduce it and prevent further buildup, and that nasty chlorine smell.

Testing Pool Water for Chloraminesequipment to test pool water for free and total chlorine, for the purpose of determing when to shock the pool.

GOOD: The Aquachek 7 Way Test Strips are a very easy product to use, and will quickly give you a free and total chlorine reading. Subtract Free from Total, and you have Combined chlorine level.

BETTER: The Taylor Troubleshooter Test Kit is a liquid DPD reagent kit, that also tests for pH, Acid Demand and Alkalinity. It’s the perfect pool test kit, and is more accurate, and easier to determine the small differences between Free and Total chlorine levels.

BEST: The SafeDip Digital Tester will not only test for free chlorine – Digitally, but also tests the ORP (Oxidation Reduction Potential); a better way to measure active sanitizer levels, and most accurately know when to shock the pool.

Shock the Pool! Pool Shocking Tips

Shocking will oxidize, or burn off, chloramines in the water so that free, available chlorine can work to destroy bacteria and algae. You can choose to shock by using a chlorine based shock like our Super Pool Shock, or use a pure oxidizer like our Chlorine-Free Shock.

n1-red-3When you shock the pool, you will need to add about 6 times as much chlorine as you usually do and maintain that high level for several hours. If you do not add enough chlorine, the chloramines will not be destroyed, and actually, more will be produced.

n2-red-3Chlorine free shock (chemically known as potassium monopersulfate) is easy to use, and will achieve the desired effect when you follow the dosing recommendation of 1lb. per 10,000 gallons (always add a little more rather than a little less).

n3-redWhichever shock you choose, when adding to the pool you should dilute the shock in a bucket of water and distribute it evenly around the pool. It is especially important to dilute chlorine based shock, to avoid the risk of bleaching a vinyl pool.

n4-redIt is best to shock at night because the sun and the heat of the day can deplete the power of the shock and because the pool will be undisturbed for many hours, giving the shock a good amount of time to circulate throughout the pool.

If you would like more information about the different types of pool shock that are available, check out Lauren’s post “The Many Types of Pool Shock”.

Jackie Wolski
InTheSwim Staff Blogger


When to Shock your Pool — 4 Comments

  1. BEST: The SafeDip Digital Tester will not only test for free chlorine – Digitally, but also tests the ORP (Oxidation Reduction Potential); a better way to measure active sanitizer levels, and most accurately know when to shock the pool.

    Based on your comment above how does one use the ORP level to determine when to shock?

    • Hi Jason, the SafeDip tester will display ORP readings from 500mA to 800mA. Anything below 600 mA is a good indication to shock, or superchlorinate the pool. ORP does not measure chlorine levels, but monitors the level of oxygen in the water, as a proxy for chlorine levels. The pH of the water must also be considered when using ORP for water analysis, as varying levels of pH produce different efficacy of the chlorine, or hypochlorous acid. If that was hard to read, it gets worse! ORP is a complicated subject, and is not foolproof, and probably not necessary for the average pool. Public pools use ORP readings as a double check of their chlorine sanitizing effects, because sometimes even though you have a good chlorine reading, chlorine demand is still higher.