Green pool? Don’t just shock it, fix the cause
Why pools go green, the 6-step recovery sequence, and the chemistry change that stops it happening again. Most green pools are CYA problems dressed up as chlorine problems.
Last updated: 2 June 2026
If your pool is green this morning and “balanced” last week, you don’t have a maintenance schedule problem, you have a chemistry imbalance that pushed past sanitary chlorine for long enough for algae to bloom. This guide walks through diagnosing the cause, fixing it correctly, and preventing the next occurrence.
The most important point up front: green water is almost never a “needs more shock” problem alone. Shocking treats the symptom. The cause is usually cyanuric acid (CYA) too high relative to free chlorine, or a hidden organic load that consumed all your sanitiser. Treat the cause as well as the symptom and the pool stays clear; treat only the symptom and it goes green again within two weeks.
Why pools go green
Algae require three things to bloom: water, sunlight, and an absence of effective chlorine. The first two are always present in an outdoor Australian pool. The third, effective chlorine, is what your maintenance routine controls. When effective chlorine drops below the algae-prevention threshold for more than 24 hours, algae bloom. The bloom is fast: a pool can go from “barely tinted” to “fully green” in 48 hours.
Effective chlorine is different from measured chlorine. Your test strip might read 2 ppm, but if your CYA is at 100 ppm, only about 1% of that chlorine is in the active hypochlorous-acid form. That’s well below the threshold algae need to be suppressed.
Diagnose first, treat second
Before you shock the pool, do four tests:
- Free chlorine (FC). If it’s below 1 ppm, you have a chlorine shortage. Possible. But check the others before concluding.
- Cyanuric acid (CYA). If it’s above 80 ppm, your effective chlorine is artificially low even at “balanced” readings. This is the cause in about 70% of residential green-water cases.
- Combined chlorine (CC = Total Chlorine − FC). If CC is > 0.5 ppm, you have a sanitiser-demand problem (organics in the water consuming chlorine faster than you can add it).
- pH. Above 8.0, even high free chlorine works poorly because the active form (hypochlorous acid) is less prevalent.
The fast diagnosis: CYA > 80 ppm + low FC = the classic green pool. The pool isn’t under-chlorinated by the test strip’s standards. It’s under-chlorinated by the chemistry’s standards. Fixing it requires either dropping CYA (drain) or significantly raising FC to compensate.
The fix sequence
Step 1: Fix pH first
If pH is above 7.6, drop it to 7.2–7.4 before adding more chlorine. Active chlorine is roughly 2× more effective at pH 7.2 than at pH 8.0. The same chlorine dose does twice the work at the lower pH. Use muriatic acid (32% hydrochloric), roughly 200–400 mL for a 50,000 L pool at pH 8.0, depending on alkalinity.
Step 2: Brush and vacuum
Before shocking, physically disrupt the algae. Brush all walls, the floor, steps, and corners with a stiff pool brush. Then vacuum to waste (bypass the filter) if you have lots of settled debris, otherwise vacuum to filter. The brushing kills the biofilm protecting the algae and exposes the cells directly to chlorine. Without this step, the shock will take 2–3× as much chlorine to be effective.
Step 3: Calculate the correct shock dose
The right shock target is the higher of:
- 10 × combined chlorine reading, or
- 40% of CYA reading
For a typical green-pool scenario, CYA 80 ppm, CC 1 ppm, shock target = max(10 × 1, 80 × 0.40) = 32 ppm free chlorine. For a 50,000 L pool, raising FC from current (probably 0–1 ppm) to 32 ppm requires about 12.5 L of 12.5% liquid chlorine. Add over 30 minutes through the return jets with the pump running.
If your CYA is above 100 ppm: the shock dose becomes impractical. 40% of 100 ppm = 40 ppm FC. To raise a 50,000 L pool to that level is about 16 L of liquid chlorine, and maintaining it for the 24+ hours needed to fully clear algae is functionally impossible. Drain and partial-refill first. Then shock.
Step 4: Run the pump 24+ hours
Filtration removes the dead algae after shocking. Without continuous filtration for 24–48 hours, the pool stays cloudy even after the algae is killed. Backwash or clean the filter twice during this period, algae loads filter cartridges fast.
Step 5: Re-test at 12 and 24 hours
At 12 hours, FC should still be near the shock level if the dose was correct. CC should be dropping toward zero. If CC is still > 0.5 ppm at 12 hours, the shock didn’t fully break, and you need another dose.
At 24 hours, FC should be holding steady (probably reduced by half due to demand and UV) and CC should be below 0.5 ppm. The pool should be visibly clearer. Greenness should be gone or fading rapidly.
Step 6: Hold FC at shock level until clear
Keep adding chlorine to maintain FC at the shock-target level until: pool is fully clear AND CC stays below 0.5 ppm overnight AND no further chlorine demand is visible. Typically 2–4 days for a mild green; up to a week for severe green.
Why your pool will go green again if you only shock
This is the part most pool owners miss. Shocking kills the current algae bloom, it doesn’t fix the underlying cause. If you started with CYA at 100 ppm and FC at 2 ppm (insufficient at that CYA), shocking back to clear pool gets you to clear water. But by next month, CYA is still 100 ppm and FC will drift back to “1–3 ppm” if you’re following standard chlorine product instructions. The pool goes green again, often within 2–4 weeks.
The permanent fix: after shocking, decide on the right ongoing FC target. If CYA is 60 ppm, your daily FC target is 60 × 0.115 = 7 ppm, with a minimum of 4.5 ppm. If you’ve been dosing for “1–3 ppm” without checking CYA, the new target is dramatically higher, which is exactly why the old routine wasn’t enough.
Different shades of green, different causes
| Colour | Most likely cause | Treatment |
|---|---|---|
| Pale green tint, water still clear-ish | Early algae bloom from low effective chlorine | Standard shock at correct level |
| Bright green, visibility still good | Established bloom, chlorine consumed rapidly | Brush + shock at higher dose. Multi-day hold. |
| Dark green / opaque green | Severe bloom + dead organic load | Brush + vacuum to waste + multi-day shock cycle. Backwash filter twice daily. |
| Green-black, slime on walls | Mature biofilm + black algae | Brush aggressively with stainless steel brush + algaecide + multi-day shock at 2× normal. Often requires partial drain. |
| Yellow-green, dust on bottom | Yellow/mustard algae (different species, more chlorine-resistant) | Shock at 3× normal level + specific yellow algae treatment + brush every 2 hours during shock cycle |
Preventing the next green pool
- Test CYA monthly, not annually. CYA above 80 ppm is the most common cause of “balanced” pools going green.
- Set your FC target based on your CYA, not based on the back of your test kit. Use 11.5% of CYA as your daily target.
- Switch off trichlor tablets once your CYA is in the 30–50 ppm range. Use liquid or cal-hypo for ongoing chlorination.
- Test combined chlorine weekly. CC creeping above 0.5 ppm is your early warning that demand is outrunning supply.
- After heavy storms or pool parties, shock as a precaution. Don’t wait until you see green.
Sources & references
- Trouble Free Pool, ABCs of Pool Water Chemistry
- Richard Falk, The Chlorine/CYA Relationship
- ANSI/APSP-11, Water Quality Standards in Public Pools
- CDC Healthy Swimming guidance
Frequently asked questions
Why does my pool keep going green even though I’m adding chlorine?
Almost always a cyanuric acid (CYA) problem. If your CYA is above 80 ppm, your measured free chlorine reading dramatically overstates how much active chlorine is actually working. Test CYA and recalculate your target chlorine as 11.5% of your CYA reading, usually much higher than the ‘1–3 ppm’ on test kit charts.
How long does it take to clear a green pool?
2–4 days for a mild green, up to a week for severe green or biofilm cases. Speed depends on three things: pool size relative to filter capacity, severity of bloom, and whether the underlying chemistry (especially CYA) gets corrected. Just shocking without addressing CYA typically returns to green within a month.
Should I drain a green pool?
Only if your CYA is above 100 ppm or your calcium hardness is above 500 ppm. If both are in normal range, draining isn’t needed, proper shocking and filter run will clear it. If CYA is high enough that shocking is impractical (40% of CYA exceeds 30 ppm and you can’t sustain that level), then drain and refill is the better option.
Will I need to replace pool water that’s gone green?
Usually not. Dead algae filters out during the recovery period (24–48 hours of pump runtime after shocking). The water itself stays, it just clears as the bloom is killed and filtered. The exception is severe long-term green-pool neglect where dissolved solids climb above 3,000 ppm above fill water, that case justifies a drain.
Can I swim in a green pool?
No. Green water indicates inadequate sanitisation, which means harmful bacteria are present even if the visible problem is algae. Wait until: pool is clear, free chlorine is in safe swimming range (typically 4–10 ppm at most CYA levels), combined chlorine is below 0.5 ppm, and the water has held those parameters for 24 hours.
What’s the difference between green algae and yellow/mustard algae?
Green algae is the common species, bright green, suspended in water and on walls, susceptible to standard chlorine shocking. Mustard/yellow algae is a different species, yellowish, settles in shaded areas, more chlorine-resistant. Mustard algae needs 3× normal shock concentration and specific treatment. If your shock at correct levels isn’t clearing the green within 48 hours, suspect mustard algae.
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