Green pool water: what to do tonight

Has the water turned green overnight? It's algae, and it can be fixed. It usually takes 2–4 days for an above-ground pool and 3–7 days for a very green in-ground pool. The secret isn't throwing in as many products as possible, but doing it in the right order with the right chlorine.

Why the water turns green

Algae take hold when free chlorine drops below the minimum for a few hours. The most common causes:

• Too little filtration. In above-ground pools the pump runs 2–3 hours when it needs 8–12.
• High pH. At pH 7.8 chlorine works at half strength.
• Too much stabiliser. The most common case for anyone using tablets all summer. The strip says "chlorine OK", but the chlorine is there and locked up.

The procedure, step by step

1. Clean up and run the filter. Remove leaves and debris, brush the floor and walls. Rinse the cartridge or backwash the sand filter.

2. Bring the pH to 7.2. At this value chlorine is at its most effective. With granular pH minus (sodium bisulphate) you need about 90 g per 10 m³ to lower it by 0.1, if alkalinity is around 100. Don't drop it by more than 0.4 at a time: wait 30 minutes with the pump running and retest.

3. Measure the stabiliser (cyanuric acid). It decides how much chlorine you really need. If you've used tablets for more than a month and never measured it, it's probably high. A strip that reads it costs about 8 euros.

4. If the stabiliser is above 90: replace some water first. Otherwise the shock is wasted. How much to drain to get down to 40:

Do it in rounds, never all at once. In above-ground pools at most half the water at a time, in in-ground pools a third: a liner pool drained too far risks the liner coming away and creasing, and with a high water table an empty pool can lift. After each round refill, retest and decide on the next.

5. Shock in the evening. The chlorine level to reach depends on the stabiliser:

Use unstabilised chlorine: liquid chlorine in a drum or calcium shock. Tablets dissolve over days and add more stabiliser, so they make the problem worse.

Example: Intex Prism Frame 457×107 (about 14,600 litres), free chlorine 1, stabiliser 40. You need to raise it by 15 points: about 1.5 L of 15% liquid chlorine, or about 340 g of 65% calcium shock.

6. Hold the shock level. Pump running day and night. On the first day test every 2–4 hours and top up every time it drops. Normal strips can't read values this high: you need a drop test kit. Brush every day.

7. When you're done. You stop when all three of these are true:

• the water is clear and you can see the floor sharply;
• overnight the chlorine drops by less than 1 point;
• combined chlorine is below 0.5.

A grey dust settles on the floor: it's dead algae. Vacuum it to waste (with a sand filter) or rinse the cartridge often.

8. Swim only with free chlorine below 10.

The green water calculator works out every dose for your pool.

Safety

• Never mix two products together, not even in the same bucket.
• The product goes into the water, never water onto the product.
• Chlorine and pH minus at different times and in different places: together they make toxic gas.
• Trichlor tablets and calcium chlorine never in the same skimmer or container: they can catch fire.

Let Serenpool do the maths

Enter your readings and your pool: Serenpool works out the doses with the products you have, tells you in which order to add them and guides you day by day until the water is clear.

Doses are calculated from the percentages stated on labels. Always check your product's label and retest after every addition.