How to fix a green pool

A green pool is algae, and the fix is chlorine held high enough, for long enough. You raise free chlorine (FC) to shock level, about 40% of your stabilizer (CYA) reading, and keep it there until three tests pass: combined chlorine at or below 0.5 ppm, no more than 1 ppm of chlorine lost overnight, and water clear with no visible algae. At CYA 40, shock level is 16 ppm.

Use liquid chlorine (or plain unscented bleach) for this. Cal-hypo and dichlor add calcium or CYA with every dose, and a green pool takes many doses.

Shock level by CYA

CYA (ppm)	Shock level (FC, ppm)	Mustard algae level (FC, ppm)	Normal target afterwards (hand-dosed)
20 or less	10	12	3 to 5
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

Above CYA 70, lower CYA before you start: the shock level becomes impractical to hold. The CYA guide shows how much water to replace.

How to clear a green pool, step by step

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.
2. Bring pH to 7.2 to 7.5. If it is higher, add acid separately, with the pump running, then wait 30 minutes before any chlorine. The pH guide has the doses.
3. Turn off the salt cell, if you have one. Liquid chlorine does the work while you hold shock level.
4. Raise FC to shock level with liquid chlorine. Add it separately, with the pump running, poured slowly in front of a return jet.
5. Brush and keep the pump running. Brush walls, floor and steps so the chlorine reaches the algae, and run the pump around the clock.
6. Test FC three to four times a day and top back up to shock level each time. Test as often as every 2 hours while it is falling fast, and always top up at dusk so it lasts the night.
7. Do the overnight test each night. Test FC after sunset and again before sunrise; losing 1 ppm or less means you passed that test.
8. Stop when all three tests pass. Then let FC fall back to the normal target for your CYA.

A worked example

A 15,000-gallon pool at CYA 40 reads FC 2. Shock level is 16, so it needs 14 ppm.

• Dose: 14 ppm × 15,000 gal ÷ 781.8 = 268.6 fl oz of 10% liquid chlorine
• In the units you buy: 2 gal, then top back up after each test

In litres: a 50,000-litre pool at CYA 40 and FC 2 needs about 5.6 L of 12.5% liquid chlorine to reach 16 ppm.

How you know it's done

All three, not one or two:

• combined chlorine (CC) at or below 0.5 ppm,
• 1 ppm or less of free chlorine lost overnight,
• water crystal clear, with no visible algae, dead or alive.

Clear water can still fail the other two. Tiny amounts of living algae keep using chlorine long after the green is gone, so keep holding shock level until all three pass.

Questions people ask

How much chlorine does it take to clear a green pool?

There is no single number. You reach shock level (16 ppm at CYA 40) and keep topping back up to it, for as many days as the algae takes. A light haze of green needs far less than a pool you can't see into.

Can I just use a bag of shock?

Not for this. Bagged shock is usually cal-hypo or dichlor. Each dose adds calcium or CYA along with the chlorine, and holding shock level takes many doses, so those side effects stack up. Liquid chlorine adds chlorine and nothing else.

Can we swim while I'm holding shock level?

Yes, if you can see the bottom of the deep end and free chlorine is between the normal minimum for your CYA and shock level. Otherwise keep everyone out.

What about mustard algae?

Hold free chlorine at 60% of your CYA for 24 hours instead: 24 ppm at CYA 40. Then carry on at the normal shock level until all three tests pass.

Do I need to lower CYA first?

Only if it is above 70 ppm. Replace enough water to bring it to 70 or lower, then start.

For your own numbers, set the goal to shock level in the chlorine calculator, or enter your test results in the water check for an ordered plan.