Green Pool Water: How to Get It Clear Again in 6 Steps

Almost every pool owner has had this morning. You walk out the back and the water that was clear yesterday has turned green — sometimes a pale milky green, sometimes dark enough that you cannot see the floor.

The usual first move is to tip in a lot of chlorine and wait. Sometimes that works. More often the water stays green, the chlorine is wasted, and three days later it goes green again.

The reason is that green water is a symptom, and it has several possible causes. Treat the wrong one and the chlorine you add cannot do its job.

Why the water turns green

The green you can see is algae that has multiplied enough to be visible. Algae needs three things at once to grow: sunlight, food, and chlorine that has stopped working properly.

Thailand has sunlight all year, so that one is fixed. What you can control is removing the food and getting the chlorine back to full strength.

Step 1: always test before you add anything

Do not put anything in the pool until you know where the water sits. At minimum you need pH and chlorine. Use a test kit or test strips that you dip and read straight away.

You are aiming for pH 7.2-7.6 and chlorine 1-1.5 ppm.

Step 2: correct the pH before you add chlorine

This is the most important step and the one most often skipped. Chlorine sanitises best when pH sits between 7.2 and 7.6. Above pH 8, the chlorine you add works at a small fraction of its potential.

If pH is high, use pH Minus. If it is low, use pH Plus. Add it in portions, run the pump so the water circulates, and retest before the next dose.

Step 3: shock the pool

Once pH is in range, shock the water. Use Shock Chlorine, which dissolves quickly and hits harder than the chlorine you use week to week.

Shock in the evening or after dark. Sunlight destroys a large share of the chlorine if you dose during the day. Leave the pump running overnight so the water turns over completely.

If you want to be back in the water sooner, Shock and Swim is formulated for exactly that.

Step 4: deal with the algae on the walls

Algae floating in the water dies from the shock. Algae gripping the walls and grout lines usually survives, and that part needs effort rather than chemistry. Scrub with a pool wall brush, or a stainless algae brush where it has really taken hold.

Then dose algaecide at 200 ml per 10,000 litres. If your pool already has metal staining, choose the metal-free formula instead so you are not adding to the problem.

Step 5: drop the debris and vacuum it out

Once the algae is dead the water usually turns from green to a grey or milky cloud. That is dead algae in suspension, and it needs to settle so you can remove it.

Use Pool Floc at 400 g per 10,000 litres, raising pH to 8.2 first for the best result. If you want it faster, Fast Floc works in 3-6 hours.

Turn the pump off, let everything settle, then vacuum it out with a vacuum head on a telescopic pole. The important part: set the valve to waste, not filter, or the fine sediment will pass through the filter and return to the pool.

Step 6: stop it coming back

If the water keeps going green despite normal care, there are usually two culprits.

The first is phosphate, which is what algae feeds on. It arrives with rain, leaves, sunscreen and the mains water in some areas. While the food is there, the algae keeps returning. Test with phosphate test strips and treat with Nil-Phos.

The second is chlorine burning off in the sun. An outdoor pool in Thailand loses chlorine to UV every day. If it disappears no matter how much you add, try Sun Block or switch to long-lasting chlorine, which holds its level far better.

The short version

Test, correct pH, shock, brush and dose algaecide, floc and vacuum to waste, remove phosphate.

Follow that order and most pools come back clear within two or three days. If you have done all of it and nothing has improved, the problem is more likely the filter or circulation. Message us on LINE and our technical team will take a look with you.