Almost every guide to this tells you to add "a double or triple dose of shock" and run the filter. That advice fails constantly, for one reason: the amount of chlorine needed to kill algae depends entirely on your stabilizer level, and "double the bag" doesn't know what yours is. A pool at CYA 80 needs four times the chlorine of a pool at CYA 20 to do the same job.
The method that works is unglamorous and has a name — SLAM, for Shock Level And Maintain. Two words matter in it, and the second is the one people skip.
First: is it actually algae?
Green water is usually algae, but not always, and the treatment for the alternative is completely different.
- Algae — the water is cloudy-green and the walls feel slippery. Chlorine disappears fast when you add it. This is the common case.
- Metals — the water is clear but tinted green, often appearing right after adding chlorine, and it doesn't feel slippery. Copper from fill water, an ionizer, or a copper-based algaecide is oxidizing. More chlorine makes this worse, not better; you need a sequestrant.
- Pollen — a yellow-green film mostly on the surface, seasonal, brushes into clumps. Skim it, don't shock it.
The quick discriminator: add chlorine and see what happens. If it vanishes within hours, that's algae eating it. If it holds and the tint remains, you're looking at metals.
This is the step that decides whether the next week goes well. Your shock level is 40% of your CYA — so at CYA 100 you'd be holding 40 ppm of chlorine for days, which is expensive, hard to measure, and slow. If your CYA is above about 90, partially drain first. Replacing water before you start is faster and cheaper than SLAMing over-stabilized water, and "I shocked it for a week and nothing happened" is nearly always this. The stabilizer calculator gives the drain percentage.
Your shock level
Not a bag, not a multiple — a number, in ppm, that you hold until you're done:
Doing this at the pool?
Prints as a one-page SLAM card — your levels, the daily routine and the exit tests. No email, no signup.
| Your CYA | Hold FC at | Normal target once you're done |
|---|---|---|
| 30 ppm | 12 ppm | 4 ppm |
| 40 ppm | 16 ppm | 4.5 ppm |
| 50 ppm | 20 ppm | 6 ppm |
| 60 ppm | 24 ppm | 7 ppm |
| 70 ppm | 28 ppm | 8 ppm |
| 80 ppm | 32 ppm | 9 ppm |
Shock level is 40% of CYA. Use liquid chlorine — the shock calculator converts this into a dose for your pool volume.
The process
- Balance pH to about 7.2 first. Chlorine works better at lower pH, and it's much easier to adjust before the water is full of chlorine than during.
- Raise FC to shock level with liquid chlorine, in the evening if you can.
- Test and re-dose back to shock level — every hour or two at first, then a few times a day. Early in a bad bloom the algae will eat several ppm between tests. That's it dying.
- Brush everything daily — walls, steps, behind the ladder, the shady corners. Algae hides in a biofilm that chlorine struggles to penetrate; brushing opens it up.
- Run the pump 24/7 and clean the filter whenever pressure rises 20–25% above clean.
- Keep going until all three exit tests pass (below). Not until it looks clear.
Debris on the floor: vacuum it out, not through
If there's a visible layer of dead algae, leaves or silt on the bottom, running it through the filter is the slow way. Vacuum to waste instead — on a multiport valve, that's the "waste" setting, which sends everything straight out of the system rather than through the filter and back into the pool. It costs you water, but it removes in twenty minutes what the filter would spend two days recirculating.
Two practicalities: the water level drops as you do it, so top up as you go and don't let the skimmer suck air, and the water you're discarding is heavily chlorinated — keep it off plants and away from anywhere it will run into a watercourse. If your filter has no waste setting, vacuum normally and plan on cleaning the filter more often.
Swimming while it's running: best not to, mostly because visibility in a bloom is poor and swimmer load works against the level you're holding. The shock calculator covers when it's safe to get back in; the short version is after the three tests below, not when the water first looks clear.
Knowing when you're done
This is the part almost every other guide gets wrong. Clear water is not the finish line — a pool can look perfect and still be feeding something. You're finished when all three of these are true on the same day:
| Test | Pass condition |
|---|---|
| The water | Clear — you can see the bottom sharply |
| Combined chlorine (CC) | Under 0.5 ppm |
| Overnight chlorine loss (OCLT) | Lose no more than 1 ppm between sunset and sunrise |
The overnight test: record FC after dark, retest before sunrise. With no sunlight, any significant loss means something in the water is still consuming chlorine.
Stopping at "looks clear" is why blooms come back a week later. The overnight test is what distinguishes a pool that's clean from one that's merely clear, and it costs nothing but two test readings.
How long it actually takes
Honest ranges, because "clear your pool in 24 hours" sets people up to quit early and conclude the method failed:
| Starting point | Realistic time |
|---|---|
| Slightly hazy, faint green tint | 1–2 days |
| Clearly green, bottom still visible | 3–5 days |
| Opaque green, can't see the second step | 5–10 days |
| Black-green swamp, visible debris | 1–2 weeks, and consider a partial drain first |
There's an intermediate stage that worries people: the water turns cloudy grey or milky blue before it turns clear. That's dead algae suspended in the water, and it's a sign the process is working. Keep the chlorine up and let the filter do its job.
What speeds it up, and what doesn't
Your filter matters more than anything else you can buy. DE filters clear the fastest, cartridge next, sand slowest — and a sand filter that hasn't had the media changed in a decade will fight you the whole way. If you're on sand and clearing slowly, a fresh media change or a small dose of DE powder into the skimmer will do more than any bottle.
Brushing genuinely helps and costs nothing. Running the pump continuously genuinely helps. Clarifier and flocculant help the filter capture dead algae after the chlorine has killed it — they don't kill anything themselves, and using them instead of chlorine just makes cloudy water that stays cloudy. Flocculant in particular drops everything to the floor and then needs vacuuming to waste, which means lowering the water level; useful at the end of a bad bloom, pointless at the start.
Algaecide is mostly for prevention, not rescue. Most products are far weaker than a proper chlorine level against an established bloom, and the copper-based ones can stain your plaster and tint your water green — the exact problem you started with. Save the money for liquid chlorine.
A serious bloom can consume several times a normal week's chlorine in a day. That's not waste — it's the measure of how much algae was there. Buy more liquid chlorine than you think you need before you start, because running out halfway and letting the level drop hands the algae its recovery. This is also why the arithmetic favors prevention so heavily: the chlorine to clear one bloom costs more than a season of holding the right level.
When draining is the better answer
Occasionally the honest advice is to stop chlorinating and start draining: when CYA is very high, when the pool is more sludge than water, when there's a visible layer of debris on the floor from a season under a failed cover, or when your fill water is cheap and your time isn't. Draining brings its own risks — a liner can float and a shell can pop out of wet ground — so read the cautions on the dilution calculator before you open a valve, and never fully empty a vinyl or fiberglass pool.
Not going green again
Blooms are almost always the same story: chlorine drifted below the minimum for the pool's CYA and stayed there long enough for algae to establish. The prevention is unexciting — know your CYA, hold FC at the matching target, and test twice a week rather than once a fortnight.
If you've just spent a week clearing a pool that went green while you were "keeping chlorine at 1–3 ppm," that reading was probably fine in absolute terms and far too low for your stabilizer. That relationship is the single most useful thing on this site, and it has its own guide. Yellow or black algae are different animals — both need higher levels held longer, and black algae needs the spots scrubbed open first.