SLAM stands for Shock, Level And Maintain. It clears algae, cloudy water and combined chlorine by raising free chlorine (FC) to shock level and holding it there until the water passes three checks. It needs nothing more than plain liquid chlorine and patience.
Your shock level
Shock level depends on your stabilizer (CYA): stabilizer protects chlorine from sunlight but also slows it down, so more CYA means a higher shock level. Test CYA first if it has been a while.
| CYA (ppm) | Shock level, FC (ppm) |
|---|---|
| 20 | 10 |
| 30 | 12 |
| 40 | 16 |
| 50 | 20 |
| 60 | 24 |
| 70 | 28 |
| 80 | 31 |
| 90 | 35 |
| 100 | 39 |
Round your CYA reading up to the next row. Above 90 ppm CYA (50 in a hot tub), a SLAM gets impractical: replace part of the water first.
Step by step
- Bring pH to 7.2–7.6 before you start. Chlorine works better there.
- Raise FC to your shock level with liquid chlorine or bleach. Avoid cal-hypo, dichlor and trichlor for a SLAM: they add calcium or more stabilizer every time you dose.
- Run the pump around the clock. Brush the walls and floor every day, and clean or backwash the filter when the pressure rises.
- Test FC every few hours while you are awake and add chlorine to keep it at shock level. Don’t let it drop below.
- Do an overnight test: check FC after sunset and again before sunrise.
- Stop when all three are true: the water is clear, combined chlorine (CC) is 0.5 ppm or less, and FC dropped 1 ppm or less overnight.
- Then stop adding chlorine and let FC fall back into your normal target range.
A green pool can take several days or longer. Stopping early is the most common reason algae comes back. Swim only when you can clearly see the bottom and FC is back below shock level.
Bromine hot tubs don’t use the SLAM method: bromine isn’t stabilized by CYA, and the usual fix for a tired tub is to drain and refill.