Pool turned green overnight? Emergency triage guide
A pool that turns green overnight has a fresh algae bloom — almost always caused by free chlorine dropping below the minimum for your CYA level. The fix: brush the walls and floor, raise FC to shock level for your CYA, run the pump 24/7, and hold that FC level until the water clears. Expect 3–7 days for a complete clear, not 24 hours.
Before anything else
Do not drain the pool, do not use copper-based algaecide, and do not rely on a pool store "green pool kit." Holding FC at shock level — combined with filtration and brushing — clears almost every green pool without permanent staining or expensive chemicals.
The 5-minute triage
Before you do anything dramatic, run through this in order:
- Test FC and CYA. You need both numbers to know your shock target. A test strip won't read FC high enough — use a FAS-DPD drop test if you have one, or dilute and retest.
- Test pH. If pH is above 7.8, lower it to 7.2 with muriatic acid before shocking. High pH wastes 60–80% of your chlorine.
- Brush the entire pool. Walls, floor, steps, behind ladders. Algae clings to surfaces in a protective layer that chlorine can't penetrate without disruption.
- Look at your filter pressure. If it's high, clean or backwash now. You're about to put a lot of work through it.
- Identify your shock-level FC target (table below). This is the level you'll maintain until the pool clears.
Find your shock-level target
The right shock level depends entirely on your CYA. This is non-negotiable — under-shocking just feeds the algae chlorine without killing it.
| Your CYA level | Shock FC target |
|---|---|
| 0–20 ppm | 10 ppm |
| 30 ppm | 12 ppm |
| 40 ppm | 16 ppm |
| 50 ppm | 20 ppm |
| 60 ppm | 24 ppm |
| 70 ppm | 28 ppm |
| 80 ppm | 31 ppm |
| 100 ppm | 39 ppm |
CYA above 80? Drain first
If your CYA is above 80 ppm, the shock target becomes unrealistic and the chlorine stays locked up. Partially drain (1/3 to 1/2) and refill to bring CYA into the 40–60 range. See why high chlorine doesn't work when CYA is too high.
The shock process (3–7 days)
Use liquid chlorine (sodium hypochlorite) — it doesn't add CYA or calcium, both of which work against you. Do NOT use trichlor tablets or dichlor granules to shock — they'll spike your CYA further and make the problem permanent.
- Bring FC up to shock level. Pour liquid chlorine around the pool with the pump running. Test 30 minutes later to confirm you hit target.
- Hold the shock level. Test FC every 1–2 hours during the day. Top up whenever it drops more than 2–3 ppm below target. The algae will consume chlorine fast at first.
- Run the pump 24/7. Filtration is half the battle. Clean or backwash the filter every 8–12 hours while you're holding shock level.
- Brush daily. Brush the entire pool every morning until the water clears. Knock biofilm into the water column where chlorine and filtration can reach it.
- Watch for color change. Green → cloudy grey/white → clear. Each transition takes about a day.
How to know when you're done
You haven't actually killed the algae just because the water looks clear. Pass these three tests before stopping the shock:
- Water is crystal clear. You can see the bottom from any angle.
- CC (combined chlorine) is 0.5 ppm or below. CC measures organics being broken down — if it's above 0.5, there's still algae or organics being killed.
- Overnight Chlorine Loss Test (OCLT) passes. Test FC right at sunset, then again right at sunrise (no UV breakdown overnight). FC should drop 1 ppm or less. If it drops more, algae is still consuming chlorine in the dark.
All three must pass. If any one fails, keep going.
Why your chlorine "looked fine" yesterday
This is the most common question after an overnight green pool: "But I tested yesterday and FC was 3 ppm!" Here's what actually happened:
| What you saw | What was actually true |
|---|---|
| FC tested at 3 ppm yesterday | Your CYA was 60 ppm — minimum FC needed was 4.5 ppm. You were below target. |
| Pool looked fine all week | Algae spores were already present; chlorine kept them in check but the safety margin was zero. |
| Hot day or rainstorm | UV/dilution dropped FC below the threshold for a few hours overnight. |
| Pump was off overnight | Stagnant water + warm temperatures + low FC = explosive algae growth. |
Algae multiplies geometrically. A pool that's "almost out of chlorine" stays clear until it isn't — then it goes from clear to green in under 12 hours.
Preventing this from happening again
- Set your FC target based on CYA, not a generic number. Use the FC/CYA chart. "Aim for 3 ppm" is not a real chlorine target.
- Test FC 2–3 times per week minimum during swim season. Daily is better.
- Don't let CYA creep above 60 ppm. If you use trichlor tablets, partial drain mid-season to keep CYA in range.
- Run the pump enough hours. 8–12 hours/day minimum in summer. Skimmer needs to circulate water for filtration and chemical distribution.
- Test FC the morning after a heavy rain or pool party. Dilution and bather load both knock FC down.
Never have an overnight green pool again
PoolChem Tracker calculates your real minimum FC based on CYA, charts your readings over time, and warns you before chlorine drops below the algae threshold.
Cloudy & Green Pool Series
Related reading
- FC/CYA Chart — the chart that prevents green pools
- How to Raise Pool Chlorine Safely — dosing for liquid chlorine
- Pool Chlorine Too Low? — diagnose why FC keeps dropping
- Why Won't Chlorine Stay in My Pool? — 7 causes of disappearing FC
- Pool shock calculator — find your CYA-based shock target and exact chlorine dose
- Chlorine dose calculator — top up FC once the pool clears