SLAM stands for Shock Level And Maintain. Test CYA, drop pH to about 7.2, then raise free chlorine to 40% of your CYA reading (minimum 10 ppm) with liquid chlorine and hold it there — testing and topping up several times a day, pump running 24/7, brushing daily. The SLAM ends only when all three pass: water is clear, combined chlorine is 0.5 ppm or below, and the pool loses under 1 ppm of FC overnight.
What a SLAM Actually Is
A SLAM is not a product and not a bigger dose. It is a process you run over days: you take free chlorine up to a level that kills faster than algae can reproduce, and then you keep it there until the water proves it is done. The letters stand for Shock Level And Maintain, and the word doing all the work is the last one.
The distinction from ordinary shocking matters, because it is the reason most attempts fail. Shocking is a single event — you dump in a bag, the FC spikes, and within hours sunlight and the algae itself have eaten it back down. The bloom survives at low chlorine and regrows overnight. A SLAM removes that recovery window: the chlorine never drops back, so the algae never gets a chance to rebuild between doses.
The level is not a fixed number
SLAM level is CYA × 0.4, with a floor of 10 ppm. At 30 ppm CYA that is 12 ppm FC; at 50 ppm it is 20 ppm; at 80 ppm it is 32 ppm. Stabilizer binds most of your chlorine into an inactive reserve, so the more of it in the water, the higher FC has to sit to deliver the same killing power. This is why you cannot start without a CYA number.
A SLAM is the right tool for four situations:
- An algae bloom of any colour.
- Water that stays cloudy after the chemistry is already balanced.
- Combined chlorine that will not clear.
- Reopening a pool that was left uncovered over winter.
It is not maintenance — you do not SLAM on a schedule, and a pool that needs one twice a season has a chlorine level problem the rest of the time.
Before You Start: Four Things to Get Right
Every one of these is cheap to do and expensive to skip. A SLAM started without them can run for a week and still fail.
1. A real test kit, and a real CYA reading
You need to measure FC up to 20–30 ppm several times a day, which strips cannot do — they saturate and read "high" at everything above about 10. A drop kit with a FAS-DPD chlorine test is effectively mandatory here. It also gives you combined chlorine, which is one of your three exit tests. Get CYA from the same kit rather than a strip; strips are at their worst on CYA, and the whole target is calculated from that one number.
2. pH down to about 7.2
Lower pH means more of your chlorine is in the active form, and it buys you headroom — pH tends to climb during a SLAM. Do this before the chlorine goes in, because at 20 ppm FC a pH test reads falsely high and you will be flying blind for days. The pH calculator gives the acid dose. Do not chase pH again until the SLAM is over.
3. Brush, and remove what you can
Chlorine cannot reach algae under a layer of debris or inside a colony's protective coating. Brush the whole pool — walls, steps, behind ladders, the shady corners — vacuum heavy debris to waste if you have that option, and empty the skimmer and pump baskets. Repeat the brushing daily.
4. A filter that can run continuously
The pump runs 24/7 for the duration. Killing the algae is the fast part; removing the dead algae is what actually clears the water, and that is filtration. Check the filter now — clean a cartridge, backwash sand, and expect pressure to climb daily once the bloom starts dying.
One thing you should not do first: raise CYA. If stabilizer is already above roughly 80 ppm the SLAM level becomes impractical — 32 ppm FC and up, held for days — and a partial drain to bring CYA down is the cheaper route. Lowering CYA first shortens the whole job.
Running the SLAM
Use liquid chlorine or plain household bleach. Nothing else belongs in a SLAM: cal-hypo adds calcium hardness you cannot remove, trichlor and dichlor add CYA, which raises the very target you are trying to hit, and every pound of it stays in the water afterwards. Liquid chlorine adds only chlorine and a little salt.
Get to SLAM level in one dose — the shock calculator converts your volume, current FC and CYA into the exact amount. Pour it slowly in front of a running return, with the pump on. From that point the job is rhythm rather than arithmetic: test, top up, brush, repeat.
Test FC at least three or four times a day — morning, midday, late afternoon, before bed. A heavy bloom in full sun can burn through 5 ppm between two tests, and the hours it spends below target are hours the algae is winning. Each time you test, dose the gap back to target. This table is the arithmetic for that top-up, in liquid chlorine at 12.5%:
| Pool volume | Lost 1 ppm | Lost 3 ppm | Lost 5 ppm | Lost 8 ppm |
|---|---|---|---|---|
| 10,000 gal | 10 fl oz | 31 fl oz | 51 fl oz | 82 fl oz |
| 15,000 gal | 15 fl oz | 46 fl oz | 77 fl oz | 123 fl oz |
| 20,000 gal | 20 fl oz | 61 fl oz | 102 fl oz | 1.3 gal |
| 25,000 gal | 26 fl oz | 77 fl oz | 1.0 gal | 1.6 gal |
| 30,000 gal | 31 fl oz | 92 fl oz | 1.2 gal | 1.9 gal |
Liquid chlorine at 12.5%. Household bleach at 6% needs roughly double, at 8.25% about one and a half times. Read the label — chlorine strength varies by brand and it loses potency in storage, so an old jug delivers less than the table says.
A worked example: a 20,000-gallon pool at 40 ppm CYA is running a SLAM level of 16 ppm FC. The afternoon test reads 11 — five ppm gone since lunchtime — so it takes about 102 fl oz of liquid chlorine, most of a gallon, to get back to target. That is a normal day-one loss. When the same test starts reading 15 instead of 11, the bloom is dying and you are close to the end.
The Three Exit Tests
You stop when the water tells you to, not when it looks good or when you run out of patience. All three of these have to pass on the same day. Two out of three means keep going.
1. Overnight chlorine loss test (OCLT) — under 1 ppm
This is the real one. Test FC after sunset, then again before sunrise, with the pump running through the night and nothing added in between. With no sun in the equation, anything that disappeared was consumed by something living. A loss of 1 ppm or less means the pool is clean. A loss of 3 ppm means you still have a bloom, however clear the water looks.
2. Combined chlorine — 0.5 ppm or below
Combined chlorine is chlorine that has bonded to waste and stalled part-way through the job — the chloramines behind that "over-chlorinated" smell. A FAS-DPD kit measures it directly as the gap between total and free chlorine. It should fall steadily through the SLAM; a CC that sticks means there is still organic load in the water.
3. Water is genuinely clear
You should be able to see the main drain sharply, not just make out its shape. Expect the pool to pass through grey, milky blue or "cloudy but not green" on the way — that is dead algae in suspension waiting for the filter, and it is progress rather than a setback.
Once all three pass, stop dosing and let free chlorine drift down on its own. It is safe to swim below 5 ppm — no need to add anything to bring it down faster. Then set your normal target: the maintenance floor is about 7.5% of your CYA, so a pool at 40 ppm CYA needs 3 ppm FC as a minimum, not the generic 1–3. The chlorine calculator takes it from there.
How Long It Takes
How long depends almost entirely on how far the bloom got before you started:
| Starting state | Typical time to pass all three tests |
|---|---|
| Light bloom, caught early | 1–2 days |
| Properly green pool | 3–5 days |
| Swamp — nobody opened it last spring | a week or more |
A swamp gets there faster if you drain and refill part of the water before starting, rather than trying to chlorinate through that much organic load.
The shape of it is consistent. Day one, chlorine vanishes as fast as you add it and the water may look worse. By the middle, consumption drops noticeably and the colour shifts from green to cloudy grey. At the end the pool holds its level almost all day and clears as the filter catches up. That falling chlorine demand is the best progress indicator you have, and it moves days before the water looks right.
Two things stretch the timeline more than anything else: a filter that cannot keep up, and pausing overnight. Clean or backwash the filter whenever pressure rises about 8–10 psi over its clean reading — during a SLAM that can be daily. Read fixing green pool water for the algae-specific side of this: which type you are dealing with, and when a drain beats a treatment.
Why SLAMs Fail
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Dosing once a day
The commonest failure by a wide margin, and it is not a SLAM at all — it is shocking repeatedly. One dose each evening means the pool spends most of daylight below target, which is exactly the window the bloom needs. If you cannot test and dose three or four times a day, wait for a weekend when you can.
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Using tabs or cal-hypo to hold the level
Trichlor tabs raise CYA as they dissolve, so your target climbs while you chase it — a genuine losing race. Cal-hypo pushes calcium hardness up permanently, and at SLAM volumes that is enough to leave a hard-water pool scaling afterwards. Liquid versus cal-hypo covers the trade-off in full.
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Stopping when the water turns clear
Clear water with a failing OCLT is a pool that will be green again inside a week. The last day of a SLAM feels unnecessary and is the one that decides whether you do this again next month.
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Adding clarifier, flocculant or algaecide mid-SLAM
At SLAM chlorine levels these are mostly destroyed, and what survives adds organic load that consumes more chlorine — the opposite of the goal. Flocculant is worse than useless here: it drops everything to the floor, where it has to be vacuumed to waste rather than filtered. Clear the pool first, then decide whether anything else is needed.
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Trusting a strip at 20 ppm
Test strips top out well below SLAM levels, so they read the same dark result at 12 ppm and at 30. You cannot measure a gap you cannot see, and you cannot run the OCLT at all without a kit that resolves 0.5 ppm differences.