The one thing to understand first
SLAM isn't a single big dose of chlorine. It's raising your free chlorine to a specific level set by your CYA and holding it there — testing and re-dosing several times a day — until the algae is fully gone. The maintaining is what does the work.
Why your CYA sets the level
Cyanuric acid (CYA, or stabilizer) shields chlorine from sunlight — but it also weakens how hard that chlorine works. The higher your CYA, the more free chlorine (FC) you need to get the same effect. That's why a pool can read "normal" chlorine and still turn green: the CYA has quietly climbed until that chlorine level isn't enough anymore.
For a SLAM, you raise and hold FC at about 40% of your CYA level. Test your CYA first, then find your target FC:
| CYA (ppm) | SLAM FC target (ppm) |
|---|---|
| 30 | 12 |
| 40 | 16 |
| 50 | 20 |
| 60 | 24 |
| 80 | 31 |
| 90 | 35 |
If your CYA is very high (over ~90), a SLAM needs a very high FC and a lot of chlorine — lowering CYA first with a partial drain & refill is often the smarter path. Test your CYA to be sure.
The steps
- Test and balance firstTest FC, pH, and CYA. Bring pH down to about 7.2 before you start — chlorine works better at lower pH, and you're about to add a lot of it. Confirm your CYA so you know your target from the table above.
- Brush everythingBrush the walls, floor, steps, and the corners where algae clings — before and during the SLAM. Brushing exposes the algae so the chlorine can reach it.
- Shock up to your SLAM levelAdd plain, unstabilized liquid chlorine to raise FC to your target. Do it near dusk so the sun doesn't burn it off, and run the pump 24/7 for the whole process.
- Maintain the level — the real workTest FC several times a day and top it back up to target each time. Keep FC at or near the SLAM level continuously; don't let it drift down. This can take several days. That's normal.
- Keep the filter running and clean itRun the pump continuously and clean or backwash the filter as pressure rises. Dead algae has to be filtered out, so the water often goes cloudy or gray before it clears — that's progress, not a setback.
How to know you're done
The SLAM is complete only when all three of these pass on the same day:
If any one of the three fails, keep holding your SLAM level and re-test. Stop only when all three pass together.
- Use plain liquid chlorine (sodium hypochlorite) for a SLAM — not stabilized tablets (tri-chlor/di-chlor), which keep raising your CYA and make the problem worse.
- Don't swim during a SLAM — free chlorine is well above normal.
- Keep pH around 7.2 throughout; very high FC can make pH tests read falsely.
- This guide is for chlorine pools. Bromine and spa chemistry work differently.
Download the printable one-page SLAM guide — steps, the CYA target table, and the finish checklist.
Once you're clear, keep it that way
Most algae outbreaks trace back to CYA creeping up over the season until your normal chlorine couldn't keep up. After a SLAM, watch your CYA, keep FC in the right range for it, and test regularly. If you'd rather not track it by hand, Crystal Water Monitor watches your water continuously and warns you before it slips — so you catch the drift long before it becomes another green pool.
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