How much shock does a green pool need?
More than the bag says — and more than once. The shock level that actually kills an algae bloom is set by your stabilizer: 40% of your CYA reading, held there until the water clears. That's the SLAM method — Shock Level And Maintain — and the "maintain" is the part every quick-fix guide skips:
| CYA (ppm) | Shock level (FC) | First dose, 10,000 gal* |
|---|---|---|
| 30 | 12 ppm | 0.9 gal |
| 40 | 16 ppm | 1.2 gal |
| 50 | 20 ppm | 1.5 gal |
| 60 | 24 ppm | 1.8 gal |
| 70 | 28 ppm | 2.2 gal |
| 80 | 32 ppm | 2.5 gal |
*Gallons of 12.5% liquid chlorine to go from FC 1 to shock level. Your pool's numbers come from the calculator above.
A SLAM means dosing over and over — and everything except liquid carries a passenger that compounds. Dichlor is the trap: every 10 ppm of FC adds 9 ppm of CYA, which raises your shock level as you go — you're literally moving the finish line with every scoop. Cal-hypo piles on calcium instead. Liquid chlorine adds nothing but a trace of salt, no matter how many times you dose. If cal-hypo is what you have, it'll work for a dose or two — just don't run a whole SLAM on it in hard water.
The SLAM process, start to finish
1. Test CYA and set your level. The calculator above does this. If CYA is over ~90, stop — see below. 2. Dose to shock level and hold it there. Test FC every hour or two at first and redose whatever the algae ate. Early in a bad bloom you may redose several times a day — that's the algae dying, not the method failing. 3. Help it along: run the pump 24/7, brush walls and steps daily (algae hides in the biofilm), and clean the filter whenever pressure rises. 4. You're done when all three pass: the water is clear, combined chlorine is under 0.5 ppm, and you lose no more than 1 ppm of FC overnight. Then just let FC drift down to normal — no neutralizer needed.
Chlorine only disappears two ways: sunlight, and oxidizing something. Test FC after sunset, test again before sunrise — no sun in between. If you lost more than 1 ppm, something in the water is still eating chlorine, and stopping now invites the bloom straight back. The OCLT is the difference between a pool that's clear and one that's actually clean — cloudy-free but OCLT-failing pools re-green within the week.
When not to SLAM
CYA over ~90: your shock level would be 36+ ppm — a huge amount of chlorine you'd struggle to measure and hold. Do the math on a partial drain first; SLAMing over-stabilized water is the most common way the method "fails." No FAS-DPD test: ordinary test strips and color-match kits max out around 10 ppm — you cannot run a SLAM on readings you can't take. A FAS-DPD drop kit (the Taylor K-2006 class) measures to 50 ppm in 0.5 steps and is the single piece of gear that makes this method work. The water's fine: routine weekly "maintenance shock" on a clear, OCLT-passing pool is a pool-store habit, not chemistry — a properly chlorinated pool doesn't need it.
Can you swim during a SLAM?
With FC at shock level, sit it out. The water is safe to touch — it's not safe to trust your test timing, and a bloom-in-progress pool has poor visibility for supervision anyway. Once FC falls back under your CYA's normal target ceiling and the water is clear, swimming resumes. With small kids, waiting for the finish line is the sane default.