Algae bloom recovery in Pakenham - the chemical shock protocol.
Day-by-day field protocol for a green Pakenham pool. When to manually-vacuum, when to floc-and-waste, the exact 10x shock chlorine dose for a family pool, and the AS/NZS 1838 chemistry targets you have to hit before the kids are back in.
Why a Pakenham pool blooms.
Algae arrives when free chlorine drops below roughly 0.5ppm for long enough to give the algae a foothold - usually 24-48 hours. On a Pakenham summer that’s easier than people think: a string of 32°C+ days, a couple of afternoon thunderstorms diluting the chemistry, a bather load of three kids and two friends, and a chlorinator output that was set in October when daily UV demand was half what it is in February.
The three repeat-offender root causes we see every January across Cardinia Shire pools:
- Stabiliser drift above 80ppm CYA. AS/NZS 1838 recommends 30-50ppm in outdoor pools. Above 80ppm, chlorine’s sanitising power is “locked” - you can have 3ppm on the test strip and still grow algae.
- Chlorinator output frozen on the spring setting. A cell making 6g/hr is fine in October. Through January it needs 12-14g/hr to keep up.
- Phosphate above 500ppb. Lawn fertiliser run-off into the pool zone is a buffet for algae. We see this most in Lakeside and Heritage Springs where lawn programs feed phosphate into the soil and storms wash it into the pool.
The 5-day recovery protocol.
Day 1 - Diagnose, brush, shock.
- Test chemistry: free chlorine, total chlorine, pH, alkalinity, CYA, phosphate. Phosphate above 500ppb means a phosphate remover goes in alongside the shock.
- pH first: drop pH to 7.2 with hydrochloric acid before shocking. Chlorine’s kill power is dramatically higher at the low end of the AS/NZS 1838 7.2-7.6 band.
- Brush the entire pool: walls, floor, behind ladders, around the skimmer mouth. Algae shelters in dead-flow zones - if you don’t physically disturb it, the chlorine never reaches it.
- Shock dose: 10x the normal residual target. On a 45,000L family pool, that’s ~2.5kg of 70% cal-hypo or ~9L of 12.5% liquid chlorine. Dose at dusk so UV doesn’t burn the chlorine off through the day.
- Pump runs 24h overnight. Filter pressure will start climbing within hours as dead algae loads the cartridge or sand.
Day 2 - Filter clean, re-test, decision point.
Morning test: free chlorine should be 5-8ppm. If it’s back to zero, the chlorine demand was higher than the shock delivered - re-shock at the same dose. Backwash the sand filter or pull the cartridge and hose it off; the pleats will be packed with green-grey dead algae. Re-prime, run another 12-24 hours, brush again at the end of the day.
Day 3 - Vacuum or floc decision.
Light to medium bloom: manual vacuum to the filter, then a second filter clean. Dead algae will be sitting in the deep end, the size of fine grey dust. Vacuum slowly - rushing kicks it back into suspension.
Heavy bloom: add a flocculant overnight. By morning the dead algae will have settled into a grey carpet on the floor. Vacuum-to-waste - bypass the filter, send it straight out the backwash line. You’ll lose 80-150mm of water, top up with the mains hose and re-balance salinity.
Day 4 - Polish and clarifier.
Water should now be clear but possibly slightly hazy. Add a pool clarifier (polyDADMAC) to coagulate the remaining fine particles. Run pump 12 hours, clean filter one more time. By end of day the pool should look essentially normal.
Day 5 - Chemistry retest and swim-safe.
- Free chlorine back inside 1-3ppm (AS/NZS 1838 range).
- pH 7.2-7.6.
- Combined chlorine below 0.5ppm.
- Stabiliser inside 30-50ppm - if it drifted high, partial water-out is the only fix.
- Phosphate below 300ppb if you dosed remover on day 1.
All five inside spec - swim-safe. Photograph the test panel and re-test in 48 hours to confirm chemistry is holding.
What it costs to have us do it.
Light bloom (water hazy but visible to the floor): $250-$350 all-in. Two visits, chemistry, one filter clean.
Medium bloom (cloudy, can’t see the floor of the deep end): $380-$500. Three visits across five days, two filter cleans, shock + clarifier.
Heavy bloom (opaque green, less than 200mm visibility): $520-$700. Four to five visits, floc-and-vacuum-to-waste, water top-up and re-balance.
Call (03) 9003 0108 or email quotes@pakenhampoolcare.com.au - if you can send a photo we can usually tell you which tier you’re in before we leave the depot.
Bloom-prevention questions we get every January.
Should I shock my pool prophylactically every week?
No. A weekly hard-shock on a properly-run salt pool is wasteful - you’re burning chlorine, stressing the cell, and elevating combined chlorine briefly. A monthly breakpoint shock (10x dose to clear chloramines) is plenty on a domestic pool.
Is “black spot algae” treated the same way?
No - that’s a different protocol. Black spot (Phormidium) embeds into the plaster and needs a manual scrub plus targeted algaecide before the shock works. We’ll quote that separately if we see it.
Does the pool cover go on during recovery?
Off during the shock and circulation phase - the chlorine off-gas damages cover fabric and you need surface UV to help the kill. Back on once chemistry is back to swim-safe.
Will green dye stain the fibreglass shell?
Almost never on a 5-day recovery. Stains happen when blooms sit for 4-6 weeks untouched and the algae produces a permanent organic stain. Catch it inside two weeks and the pool comes back to factory-clean.
Got a green pool right now?
Same-day call-out across Pakenham, Officer, Beaconsfield, Cardinia Lakes and Heritage Springs through summer. Send a photo, get a tier-priced quote inside the hour.