Green Pool Water

What Causes Green Pool Water?

Algae is the usual culprit behind water that turns green and cloudy. It gets a foothold when chlorine runs low, the pH is off balance or the water gets warm. When a pool has algae, chlorine has usually slipped low and phosphorus may be running high.

pH alone can matter too. Water can pick up a green tint once the pH climbs past 7.8, which is why the advice here is to keep a pH of 7.2 to 7.8.

Metals are the next big cause. Too much iron, copper, manganese or hardness can bring on a green color even when no algae is present. A green or blue-green tint most often points to copper.

Why Does a Pool Go Green Again and Again?

When a pool goes green over and over, even though chemicals are in it, the trouble usually comes down to water chemistry that has drifted off balance or chlorine that has stopped doing its job. Pouring in more of the same product seldom helps, so start with a full water test.

That test should look at free chlorine, pH, total alkalinity (what keeps the pH steady) and stabilizer, and now and then phosphates or metals too. Free chlorine means the share of chlorine that is still ready to work, and it is what your test measures. Stabilizer, also called cyanuric acid, protects chlorine from the sun.

Pool Turned Green Overnight

If a pool goes green overnight or after it rains, the reason is often a pump that is not moving the water around as it should. Algae can multiply fast in still water. Before adding anything, make sure the pump is on and water is moving through the filter.

Pool Green but Chlorine High?

If the water is green even with a high free chlorine reading, check the stabilizer and pH. When stabilizer piles up, chlorine loses its effectiveness, even with plenty showing on the test. A high pH, a built-up stabilizer level or warm water all mean the pool needs extra chlorine to get the same result.

The test itself can mislead you too. Most strips only read up to about 10 ppm (parts per million, the unit on the test), so while treating algae, use a drop test kit that measures high free chlorine. You will find these kits sold under the name FAS-DPD.

Pool Water With a Green Tint You Can See Through

Water that looks green yet stays clear enough to see through usually points to metals, and a sequestrant can tie them up. A sequestrant is a liquid treatment that latches onto the metals so they quit tinting the water. Clear green water that shows up right after a shock is usually metal as well, and more shock only makes things worse, because a shock will not clear metals out and may leave stains.

Shock Amounts by Water Color

The amounts below work for every 10,000 gallons of water in the pool. The cal-hypo figures assume powdered shock at about 65% strength. The liquid amounts assume chlorine at 12.5% strength, and a 10% bottle needs about a quarter more.

Type your pool size into the box on the home page and it works out the amounts for your pool. Buy enough shock to make up the amount your pool needs, and add just that much. Cal-hypo also puts calcium into the water. When calcium hardness already tests above 400 ppm, liquid chlorine is the better pick.

After a shock, the free chlorine test will read lower than what you put in. The algae and debris burn through the chlorine right away.

How Long Does Clearing Take by Shade?

Water that is only light green and caught early typically clears up within a day or two. Water that is dark green or black can need five to seven days, and sometimes more. If the water turns cloudy after a shock, it means the algae has died, and the filter will gradually clear it away.

The treatment is complete once the water runs clear and free chlorine drops by no more than 1 ppm overnight. To check it, test once the sun sets, then again before it rises. A bigger drop means something in the pool is still eating the chlorine.