Why one shock never works
Algae is not killed by a moment of high chlorine; it is killed by chlorine held above the level it can survive, for as long as it takes. Dump one bucket in on Saturday and the pool goes cloudy blue, the chlorine is consumed by Sunday, and the green returns on Tuesday looking exactly as it did. The work is in the holding, not the dosing.
Before you add anything
- Test cyanuric acid. It sets how much chlorine you need to hold, and a pool at 120 ppm is a completely different job from one at 30.
- Get pH into the WHO band of 7.2 to 7.8 — chlorine works far harder at the lower end.
- Clean or backwash the filter. It is about to do a week’s work in three days.
- Net out leaves and debris. Every organic thing in there is chlorine demand.
- Tell the client the pool is closed until it is finished, and why.
Do not swim
Free chlorine during this process sits far above the 3 mg/L ceiling WHO sets for public and semi-public pools. Nobody gets in until it has fallen back and been tested.
The sequence
| Day | Work | What you should see |
|---|---|---|
| Day 1 | Balance pH, clean filter, brush, raise chlorine to the hold level | Green goes grey-green |
| Day 2 | Test chlorine 3–4 times, top back to the hold level each time, brush, filter runs 24 h | Cloudy grey |
| Day 3 | Same again; vacuum settled debris to waste, top up the water | Cloudy blue |
| Day 4 | Hold level, clean the filter again when pressure climbs | Hazy blue, floor visible |
| Day 5 | Run the overnight loss test | Pass or keep holding |
The pump runs continuously the whole time. This is the one situation where a variable-speed schedule should be overridden — filtration is doing half the work.
The test that tells you it is over
At dusk, record free chlorine. Do not add anything overnight. Test again at first light. If the pool lost more than about 1 ppm, something is still consuming chlorine and you are not finished — hold another day. If it lost 1 ppm or less and the water is clear, the job is done.
This test is the whole reason the process ends cleanly instead of “it looks alright”. It is also the reason algae comes back for people who skip it.
After it clears
Let free chlorine fall naturally to the normal band before anyone swims — measured, not guessed. Clean the filter properly one last time. Then find the cause: nine times out of ten it is stabiliser above the ceiling, a pump run time that was quietly cut, or a month of tablets in a floater. Fix that and you will not do this again this year.