My Pool Is Green — How to Clear It (the SLAM Method)
You're seeing: Green, cloudy, or hazy water; algae on the walls or floor.
Test CYA, look up your SLAM chlorine level for that CYA, and raise free chlorine to it with liquid chlorine. Hold it there (retest and re-dose 3–4× a day), brush daily, run the pump 24/7. You're done only when all three are true: CC ≤ 0.5, overnight FC loss ≤ 1.0 ppm, and the water is crystal clear.
Green, cloudy, or hazy water is almost always algae, and there’s one method that clears it and keeps it gone: SLAM — Shock Level And Maintain. “Shock” here isn’t a bag of product; it’s a level of free chlorine you hold until the pool passes three tests.
Do this
- Test your CYA. It sets your SLAM chlorine level. If CYA is very high (over ~70), partially drain and refill to bring it down first (see how to lower CYA), or you’ll be dosing enormous amounts.
- Look up your SLAM level for that CYA (CYA 30 → 12, 40 → 16, 50 → 20, 60 → 24, 70 → 28). The calculator gives the dose in your gallons.
- Balance pH to 7.2–7.5 before you start (the pH test is unreliable at high chlorine).
- Raise free chlorine to SLAM level with plain, unscented liquid chlorine — not cal-hypo (adds calcium) or dichlor (adds CYA).
- Maintain it. Retest and re-dose back to SLAM level 3–4× a day (every couple of hours clears it faster). Turn off any salt system while you SLAM.
- Brush the whole pool daily — walls, floor, steps, behind ladders and lights, shaded corners.
- Run the pump 24/7 and clean the filter when pressure rises ~25% over clean.
What's your SLAM level?
Enter your CYA and read the SLAM line — that's the free chlorine you raise to and hold until all three exit criteria pass.
Straight off Trouble Free Pool's FC/CYA chart (minimum ≈ 7.5% of CYA, SLAM ≈ 40%), snapped to the nearest published row — the chart table stays the source of truth. Exact doses for your gallons: the chemical calculator. CYA way over 100? More chlorine isn't the fix — lowering CYA is.
You’re done only when all three are true
This is the part everyone skips — and why algae “comes back.” A blue pool isn’t a finished pool. Stop only when all three exit criteria pass:
- Combined chlorine (CC) is 0.5 ppm or lower.
- Overnight chlorine loss is 1.0 ppm or less — test FC after sundown and again before sunrise; losing more than 1 ppm overnight means algae is still feeding.
- The water is crystal clear with no visible algae, live or dead, on the floor or walls.
What NOT to do
Don’t dump one bag of “shock” and walk away — chlorine gets eaten fighting algae and drops within hours. Don’t reach for algaecide, clarifier, or flocculant as the fix; they treat symptoms and add problems. Don’t trust test strips for this — you need a FAS-DPD drop test to read the high chlorine levels accurately. And don’t quit at “looks blue” — finish the three criteria or it returns.
You need a real test kit for this
A SLAM can’t be run on strips or the little OTO/DPD block — they can’t read high FC or small overnight losses. This is the one time we’ll tell you to buy something: a FAS-DPD drop kit (the Taylor K-2006 or a TF-100 are the standards). Buy it once, use it for years.
If your chlorine and CYA look “normal” and it’s still green, read Green but my levels look fine — that’s almost always high CYA hiding the real problem. If it keeps returning specifically on shaded walls, it may be mustard algae. And dark spots that come back in the same places after you brush and shrug off a shock aren’t loose algae at all — that’s black algae, which takes a different attack.
Frequently asked
How do I get rid of algae in my pool?
Use the SLAM process: raise free chlorine to the SLAM level for your CYA (e.g. 16 ppm at CYA 40, 24 ppm at CYA 60) with plain liquid chlorine, and maintain it by retesting and re-dosing several times a day. Brush the whole pool daily and run the pump 24/7. Keep it there until all three exit criteria are met. A single dose of 'shock' does not do this — you have to maintain the level.
How long does it take to clear a green pool?
Anywhere from a couple of days to a couple of weeks, depending on how bad it is and how diligently you hold the chlorine level. The more often you retest and re-dose back to SLAM level (as often as every couple of hours), the faster it clears. It's finished when you pass all three exit criteria, not when it just looks blue.
Is one dose of shock enough to clear a green pool?
No. 'Shock' is a level you maintain, not a one-time dose. Chlorine gets consumed fighting the algae, so a single dose drops back down within hours and the algae recovers. You have to hold the SLAM level with repeated dosing until you pass the three exit criteria.