Breakpoint Chlorination and SLAM

A strong chlorine smell, murky water, or stinging eyes after a swim usually point to one thing: chloramines. These form when chlorine binds with the sweat, body oils, and organic debris swimmers bring in. The fix is breakpoint chlorination, which most people just call shocking. Add enough chlorine and it burns the chloramines off and the water clears. For a real algae problem, you step it up to SLAM (Shock Level And Maintain). Here is how both work.

What Breakpoint Chlorination Is

Breakpoint is the point where you have added enough free chlorine to oxidize all the contaminants in the water, including the ammonia and nitrogen compounds that make chloramines. Get past that threshold and the chloramines are destroyed, leaving only free chlorine. The chlorine smell goes away and the water is properly sanitized again.

Signs You Need to Shock

• Strong chlorine odor despite normal or high chlorine readings.
• Cloudy or hazy water that won't clear with filtration.
• Visible algae growth, from green blooms to black spots.
• High bather loads after pool parties or heavy use.
• Heavy rainfall introducing contaminants.

The SLAM Method (Shock Level And Maintain)

A single shock is a one-time dose. SLAM means holding the chlorine at shock level over days, not hours, until every contaminant is gone. That is what it takes for algae, badly clouded water, or stubborn organic buildup.

1. Test and Adjust pH

Get pH between 7.2 and 7.4 before you shock so the chlorine works as hard as it can.

2. Raise Free Chlorine to Shock Level

Dose up to a shock level of 10-12 ppm or higher, with the exact amount depending on how bad the contamination is and how big the pool is.

3. Maintain Shock Level

Test free chlorine (FC) and combined chlorine (CC) every few hours and re-dose to keep FC at shock level. Hold it there until CC drops below 0.5 ppm and the water clears.

4. Brush and Vacuum

Brush the walls and floor to break up algae and biofilm so the chlorine can reach it. Vacuum the loosened debris to waste so it does not re-contaminate the pool.

5. Filter Care

Run the pump the whole time. Backwash or clean the filter regularly as it loads up with dead algae and debris.

6. Finish and Balance

Once CC is near zero and the water is clear, let the chlorine drift back down to 1-3 ppm on its own. Retest pH, alkalinity, and CYA and bring them back into range before anyone swims.

Common Pitfalls and Tips

Don't Overlook Cyanuric Acid (CYA)

High CYA (stabilizer) holds chlorine back and can make a shock fall flat. If your CYA is too high, drain and refill part of the pool to bring it down before you SLAM.

Pre-Dissolve Granular Chlorine

Dissolve granular chlorine in a bucket of water first, then pour it in. Dumped straight on the floor it can bleach the liner or surface.

Test Often

A SLAM lives or dies on testing. Expect to test and re-dose several times a day to hold the chlorine at shock level.

Safety First

Keep everyone out of the water while chlorine is at shock level. Store your chemicals somewhere cool and dry, and handle them carefully.

The Bottom Line

Breakpoint chlorination and SLAM are how you reset a pool that has gotten away from you, whether the problem is chloramines, murky water, or algae. The work is mostly dosing and testing: add enough chlorine, hold it there, and keep testing until the numbers come back. Do that and the water clears.

Frequently Asked Questions

How much chlorine do I need to shock a pool?

Raise free chlorine to a shock level of 10 to 12 ppm or higher. The exact dose depends on how bad the contamination is and how big the pool is, so test and re-dose to hold that level rather than guessing a single amount.

How long after shocking can I swim?

Do not go by a fixed number of hours. Wait until free chlorine drops back down to 1 to 3 ppm on its own, which is often the next day. Keep everyone out while chlorine is still at shock level.

Why won't my pool hold chlorine?

Usually it is high cyanuric acid or an active contaminant like algae eating the chlorine faster than you add it. High CYA holds chlorine back, and above roughly 80 ppm the dose needed to reach breakpoint becomes impractical, so lower CYA by replacing some water before you SLAM.

What is the difference between shocking and SLAM?

A shock is a one-time dose. SLAM (Shock Level And Maintain) means holding chlorine at shock level over days, testing and re-dosing several times a day, until combined chlorine drops below 0.5 ppm and the water clears. SLAM is what it takes for algae or badly clouded water.