Free tool No. 4
How to fix a green pool
Raise free chlorine to shock level and hold it there until the water passes three checks. Shock level depends on your stabiliser (CYA): at CYA 40 it is 16 ppm. For example, a 50,000-litre pool at 2 ppm needs 5.5 L of 12.5% liquid chlorine to get there, then a top-up to 16 ppm after every test. Fix pH first (7.2 to 7.5), and stop when combined chlorine is 0.5 ppm or less, chlorine drops 1 ppm or less overnight, and the water is clear.
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Worked example
A green 50,000-litre pool at CYA 40, with FC 2 ppm, pH 7.4 and 12.5% liquid chlorine.
- 1
Find shock level from CYA
CYA 40: 0.393 × 40 + 0.5 = 16.22, rounded down to 16 ppm
- 2
Check pH first
pH 7.4: within 7.2 to 7.5, so nothing to fix first
- 3
Work out the increase
16 ppm − 2 ppm = 14 ppm
- 4
Convert the volume
50,000 L ÷ 3.785 = 13,209 US gal
- 5
Dose the 12.5% chlorine
FC 2 → 16 ppm: 14 ppm × 13,209 gal ÷ 977.9 = 189.1 fl oz
- 6
Round to what you can pour
189.1 fl oz = 5.59 L, rounded to 5.5 L
Add 5.5 L of 12.5% liquid chlorine, then add chlorine back to 16 ppm each time you test.
Pour it slowly in front of a return jet with the pump running. Swim only when you can see the bottom of the deep end and chlorine is between 3 and 16 ppm.
Shock level by CYA
| CYA (ppm) | Shock level (SLAM) | Mustard algae | Normal range afterwards |
|---|---|---|---|
| 30 | 12 | 18 | 4 to 6 |
| 40 | 16 | 24 | 5 to 7 |
| 50 | 20 | 30 | 6 to 8 |
| 60 | 24 | 36 | 7 to 9 |
| 70 | 28 | 42 | 8 to 10 |
Shock level is about 40% of CYA, and 60% for mustard (yellow) algae. Above CYA 70, replace water to bring it to 70 or lower before you start. Below 30, add stabiliser first. The normal range is for chlorine added by hand.
12.5% liquid chlorine to reach shock level
| Pool size | CYA 30 (12 ppm) | CYA 40 (16 ppm) | CYA 50 (20 ppm) | CYA 60 (24 ppm) | CYA 70 (28 ppm) |
|---|---|---|---|---|---|
| 20,000 L | 2 L | 2.5 L | 3.25 L | 3.75 L | 4.5 L |
| 30,000 L | 3 L | 3.75 L | 4.75 L | 5.75 L | 6.75 L |
| 40,000 L | 3.75 L | 5 L | 6.5 L | 7.75 L | 9 L |
| 50,000 L | 4.75 L | 6.5 L | 8 L | 9.5 L | 11 L |
| 60,000 L | 5.75 L | 7.75 L | 9.5 L | 11.5 L | 13.5 L |
| 70,000 L | 6.75 L | 9 L | 11 L | 13.5 L | 15.5 L |
Then top back up to shock level after every test. Weaker chlorine needs more: pick its strength in the calculator.
How to clear a green pool
- 1
Test CYA, pH and free chlorine
CYA sets the shock level. If it is above 70, replace water to bring it to 70 or lower first. If it is below 30, add stabiliser first.
- 2
Fix pH first
A shock works best with pH between 7.2 and 7.5. If it is higher, add pool acid (hydrochloric) with the pump running, poured slowly in front of a return jet, then wait 30 minutes before any chlorine. If it is lower, the calculator gives the dose to raise it to 7.5. Never let acid and chlorine touch.
- 3
Raise chlorine to shock level
Add liquid chlorine on its own, with the pump running, poured slowly in front of a return jet. If you have a salt chlorinator, turn it off until the rescue is finished.
- 4
Hold it there
Test chlorine at least 3 or 4 times a day (every 2 hours if you can) and add chlorine back to shock level each time. Brush the walls and floor daily, run the pump around the clock, and clean the filter when its pressure rises.
- 5
Test overnight
Test chlorine at dusk and again before sunrise. Losing 1 ppm or less overnight passes.
- 6
Stop when all three checks pass
Combined chlorine is 0.5 ppm or less, the overnight loss is 1 ppm or less, and the water is crystal clear with no visible algae, dead or alive. Then let chlorine fall back to the normal range for your CYA.
How we calculate
Shock level comes from your stabiliser (CYA) reading: 0.393 × CYA + 0.5, rounded down, and never below 10 ppm, which is about 40% of CYA. It is the formula behind the published chlorine and CYA chart that pool-chemistry communities use, and the level the app holds during an algae rescue.
The first dose is ppm × US gallons (your litres, converted) ÷ a factor for your chlorine’s strength, rounded to an amount you can measure. The order of the steps and the three checks for when it’s done come from the same chemistry engine as the app: keep swimmers out while chlorine is too low to kill germs, lower CYA above 70, get pH between 7.2 and 7.5, add stabiliser below 30, then shock.
Its limits: the dose takes you to shock level once, and how many top-ups and days the rescue takes depends on the algae. Liquid chlorine loses strength in storage, so chlorine that has been stored a while needs more. The acid dose for pH assumes alkalinity of 70 ppm, because the planner doesn’t ask for it. It assumes an outdoor pool with a plaster finish; the drain advice for vinyl and fibreglass pools is more cautious.
Questions
How long does it take to clear a green pool?
There is no set number of days: it depends on how much algae there is. Keep chlorine at shock level (16 ppm at CYA 40) until all three checks pass: combined chlorine is 0.5 ppm or less, overnight chlorine loss is 1 ppm or less (test at dusk and again before sunrise) and the water is crystal clear with no visible algae, dead or alive. Water can look clear and still fail the overnight test, because something in it is still using up chlorine.
Can I use granular chlorine instead of liquid?
Not for a rescue. Dichlor adds about 0.91 ppm of stabiliser for every 1 ppm of chlorine, so the first dose in the example above (2 to 16 ppm) would also add about 12.7 ppm of CYA and raise shock level to 21 ppm. Cal-hypo adds about 0.71 ppm of calcium for every 1 ppm, and a rescue takes many doses. Liquid chlorine adds neither.
Do I need to lower my stabiliser (CYA) first?
Only if it is above 70 ppm. The rescue needs stabiliser at 70 or lower, so replace enough water to get there first; the calculator works out how much. Below 30 it adds stabiliser first, because without it sunlight burns off chlorine within hours.
Can we swim during the rescue?
Only when you can see the bottom of the deep end and free chlorine is between the minimum for your CYA and shock level: 3 to 16 ppm at CYA 40. Otherwise keep everyone out.
What about mustard (yellow) algae?
It needs a higher level, about 60% of CYA: 24 ppm at CYA 40. For mustard algae, keep chlorine at 24 ppm for at least 24 hours before letting it ease to 16 ppm. Then carry on at shock level until all three checks pass.