SLAM – Shock Level and Maintain

SLAMing your pool is the best way to eliminate algae, sanitize your pool and get your water crystal clear. If you have algae, or the CC level is above 0.5, or the free chlorine (FC) level is zero, you should SLAM the pool. You cannot do this overnight…it often takes several days to see it completed…no shortcuts. If you are maintaining proper FC levels there is no need to SLAM (or shock) weekly, or on any other regular schedule.


  • You need to have a FAS-DPD chlorine test
  • You also need to know your current CYA level, and then look up the corresponding FC shock level. You can use the The Chlorine/CYA Chart at Pool School to find the correct FC shock level for your CYA level.
  • It is best to use bleach or liquid chlorine when SLAMing. You will want to have enough on hand to raise the FC level to shock level at least a couple of times. It is especially important to avoid using dichlor when SLAMing, as it will raise CYA far too quickly.
  • Make sure the pool is free of leaves and other debris (at least as much as possible).
  • Make sure your filter is working properly and you understand how to clean it out should it become necessary.
  • Check and adjust the PH to between 7.2 and 7.5. The PH test isn’t reliable during SLAMing so make sure to take care of this before you start.
  • Run the pump 24/7 until you are done SLAMing


  • Test the FC level and add enough chlorine to bring FC up to shock level (see here for correct shock level)
    Test and adjust chlorine levels as frequently as practical, but not more than once per hour, and not less than twice a day. Chlorine additions should be frequent, especially at the beginning. Algae and other organic debris will consume chlorine very rapidly at first. As things progress, you will lose less chlorine each cycle and can add chlorine less frequently.
  • Brush and vacuum the entire pool once a day
  • Backwash or clean the filter as needed
  • Vacuum up debris as needed

You are done when:

  • CC is 0.5 or lower
  • AND you pass an OCLT (i.e. overnight FC loss test shows a loss of 1.0 ppm or less)
  • AND pool water is crystal clear with no visible sign of algae (dead or alive) on floor and walls

When all three are true, you are done SLAMing and can allow the FC to drift down to normal levels.


If you have questions that aren’t covered here, by all means ask on the forum…you’ll get lots of help.

It is very difficult to do this procedure correctly unless you are willing to do your own testing with a FAS-DPD chlorine test (included in the better test kits and also available separately). Without that specific test, you risk wasting your time and potentially having to start all over later.

SLAMing an outdoor pool is most effective when CYA is around 30 to 40. Below 30 you lose too much chlorine to sunlight. As CYA goes up, SLAMing requires more and more chlorine, which starts to get impractical around 80 or 90. With CYA above 90 we recommend replacing water to get CYA down before you start the SLAM process.

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

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

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

Higher FC levels will oxidize contaminates more quickly, but levels that are too high can cause damage to the pool or the pool equipment. 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 just how much chlorine will be required to SLAM the pool. Instead, chlorine is added to maintain shock level until testing shows that the process is complete.

If you have a salt water chlorine generator, chlorine tablet dispenser, injection pump, or Liquidator, you will still need to use another chlorine source for SLAMing. All of these devices are designed to add chlorine slowly and steadily over many hours. To kill algae or lower CC, you need to put in lots of chlorine all at once. 

The best solution to this is simply shut off the SWG, Liquidator, injection pump, etc. and use liquid chlorine only for the duration of the SLAM

If you have a sand filter, you can speed up the process of clearing up the water by adding a little DE to your filter. See Add DE to a Sand Filter for more information.