A green pool almost always means one thing: your free chlorine has been too low for your cyanuric acid, and algae took over. Clearing it isn't about dumping in "shock" — it's about holding the right chlorine level until the algae is dead.
Why it went green
The more CYA (stabilizer) you have, the more free chlorine you need to keep the same killing power. If you ran 2–3 ppm at a CYA of 70–80, that's effectively too weak — perfect conditions for algae. Check your CYA-scaled chlorine target →
The SLAM method (Shock Level And Maintain)
- Test CYA and balance pH. Bring pH to ~7.5 first so the chlorine works and your test is accurate.
- Find your shock level. It's about 40% of your CYA as free chlorine. Shock calculator →
- Raise FC to that level with liquid chlorine — not tabs or dichlor, which add more CYA.
- Hold it there. Test and re-dose several times a day to keep FC at the shock level. This is the part people skip — and why pools go green again.
- Brush and filter constantly. Brush the walls daily; run the pump 24/7; clean or backwash the filter as it loads up.
How do you know it's done?
You're finished when all three are true: the water is clear, chlorine loss overnight is 1 ppm or less, and combined chlorine is near zero. Only then drop back to your normal CYA-scaled level. A bad green-to-clear can take several days — that's normal.