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Shock Your Pool

At some point, everyone will need to shock their pool. When you shock the pool, you will eliminate algae, combined chlorine (CC), and any other organic contaminant present. You will need to shock the pool if you have algae, the CC level is above 0.5, or the free chlorine (FC) level is zero. People also like to shock the pool when their FC level falls significantly, as it often does after a major storm or a large swim party. This can be done even if FC doesn’t get all the way to zero, just to be sure everything is all right. However, if you are maintaining proper FC levels there is no need to shock weekly or on any other regular schedule.

To shock properly, it is very helpful to have a FAS-DPD chlorine test. You will also need to know your current CYA level, so that you can then look up the corresponding FC shock level. You can use the The Chlorine/CYA Chart found at The Pool Calculator (www.thepoolcalculator.com) to find the correct FC shock level for your CYA level. We recommend using bleach or liquid chlorine (sodium hypchlorite) when shocking.

How To Shock:

First, measure the FC level.
Next, add enough chlorine to bring FC up to shock level or higher.
Continue repeating steps 1 and 2 as frequently as possible, but not more than once per hour and not less than twice a day, until:
CC level is 0.5 or lower;
An overnight FC loss test shows a loss of no greater than 1.0 ppm
The water is once again clear.
Brush the entire pool once a day (brushing more will not hurt, however!).
Don’t forget to backwash or clean the filter as needed.

Depending on what kind of filter you have, it can take a week or more to completely clear up the water, even after all of the algae is dead. DE filters are usually considerably faster than that, but they require frequent attention when cleaning up algae. Sand filters tend to be the slowest, while cartridge filters fall somewhere in between.

While you are shocking, the appearance of the water should improve some each day, although perhaps only by a little. If you fail to see any improvement, then you might have a problem with your filter, have a higher CYA level than you think, bad circulation, or some other more complex problem.

The reason for shocking is to add enough chlorine to oxidize all of the algae, combined chlorine, bacteria, viruses, ammonia, and other organic contaminates. Oxidization (with chlorine) breaks down the organic molecules into smaller parts, which are harmless. When you are shocking, you will need to keep adding chlorine until the breakdown process is complete.

High FC levels will oxidize contaminates more quickly, but levels that are too high could cause damage to the pool or pool equipment. The recommended shock levels are designed to break down contaminates reasonably quickly, while posing minimal risk to the pool. It is impossible to know in advance exactly how much chlorine will be required to shock your pool. Instead, chlorine will need to be added to maintain shock level until testing shows that the process is complete.

Even if you have a salt water chlorine generator, chlorine tablet dispenser, or Liquidator, you will still need to use another chlorine source for shocking. These devices are all designed to add chlorine slowly and steadily over many hours, and provide a relatively stable chlorine residual in the pool. To kill algae or lower CC, you will need to put in a lot of chlorine all at once. While the SWG, tablet dispenser, or Liquidator can be very helpful in the follow up stages to maintain FC at shock level, for the initial couple of chlorine applications you need to use another chlorine source to get to the level needed to complete the kill.