Saltwater Pool Green

Your pool is green because free chlorine was below the level your stabilizer level required, long enough for algae to take. That is almost always the answer in a salt pool, and it is usually true even when the panel is lit up green and the water tests “in range.”

Do not add salt. Do not press boost. Test first, in this order, then dose by hand.

The first hour

1. Test free chlorine and cyanuric acid. Both, from the same sample, with a kit that gives you numbers rather than a color band. You cannot make a decision about a green pool without both.

2. Compare them. The minimum free chlorine for a salt pool is roughly CYA multiplied by 5%, on the FC/CYA chart that reproduces the Trouble Free Pool methodology. At CYA 80 that is 4.0 ppm. At CYA 60 it is 3.0. If your reading is under that line, you have found your cause and can stop diagnosing.

3. Look at the cell. Kill the power, pull the cell, shine a flashlight through the plates. White crust means scale and reduced output. Clean plates are dark grey and smooth.

4. Check what the pump actually runs. Not what you think it runs. A cell makes chlorine only while water moves through it, so a schedule that was fine in May is often short in August.

5. Brush the walls, clean the filter, then shock by hand. Doses are on salt water pool shock. The generator is not part of this step.

Why a salt pool goes green differently

A chlorine pool goes green when someone stops adding chlorine. A salt pool goes green while the owner is doing everything the manual said. Three failures specific to this equipment, in the order worth checking.

The cell is not producing what the panel claims

Salt systems do not measure chlorine. They run at a percentage and assume the rest. They do not measure salt either. Most systems estimate salinity with an algorithm based on how easily current crosses the cell, so scale, plate wear, poor flow or the wrong cell model selected in the menu all move the displayed number while nothing changes in the water.

A panel that looks normal is not evidence of chlorine. Your test kit is. The full diagnostic is on salt cells and water care.

The free chlorine minimum moved and nobody told you

Trouble Free Pool is direct about the mechanism: stabilizer protects free chlorine from the sun, so the level you must hold rises as CYA rises. Let it fall too low for the CYA you are running and you get algae.

This is why “I have chlorine, why is it green” is such a common sentence in a salt pool. You do have chlorine. You do not have enough of it for your stabilizer level. Work through the CYA and chlorine chart once and the number stops being a mystery.

The pump schedule is short

Output percentage and run hours multiply. A cell at 40% for 12 hours and a cell at 80% for 6 hours make roughly the same chlorine, so cutting the schedule to save power in July cuts production when demand is highest. The arithmetic is on how to increase free chlorine in a salt water pool.

The number most green-pool owners are sitting at

Hayward’s AquaRite manual asks for free chlorine of 1.0 to 3.0 ppm. It also asks for stabilizer of 60 to 80 ppm with 80 as the target. Put those two lines side by side and do the arithmetic.

At the stabilizer level Hayward specifies, most of Hayward’s own chlorine range sits under the minimum on the chart. That is a real contradiction between two authorities and we are not going to pretend one is obviously wrong. What we will say is that a green pool is the test result. If you held 2 ppm at CYA 80 and the water went green, the fixed band was not enough for your pool, whatever the chart on the lid says.

What clearing it actually takes

Shock, held. Not one dose and a hopeful weekend.

The shock level on the same chart is about CYA multiplied by 40%, so a pool at CYA 80 is looking at 32 ppm of free chlorine, held there while the filter runs and you brush. Chlorine is consumed as it kills, so a single dose drops back within hours and the algae keeps growing. Why holding matters more than the size of the first dose is on algae in a saltwater pool.

Turn the generator down while you do it, not up. It cannot reach those levels and it wears the plates trying.

Buy liquid chlorine or cal-hypo, not tabs. Trichlor is only 45.8% chlorine by weight, and the majority of the rest is cyanuric acid. Using it to clear a green pool raises the very number that set your minimum. The short list of what to own is on salt water pool chemicals.

Things not to do today

Do not press boost and wait. In The Swim measured what it does: most salt systems in shock mode “will only raise the FC level by a few clicks - up to 5-7 ppm. It also does it very slowly, which will not have the desired effect of killing algae.” It also “places a heavy demand on the salt cell, shortening its lifespan.”

Do not add salt. Salt does not sanitize, and a low salt warning on a green pool is usually the cell, not the water. If an independent test does confirm it, size the dose with the pool salt calculator afterward.

Do not lead with algaecide. It has a role, and it is not this one. See algaecide for a salt water pool.

Do not stop when it turns cloudy. Green going to grey or milky means the algae is dying and the filter has work to do. That is progress, and cloudy salt water pool covers the rest.

Not going green again

The fix is a number, not a product. Decide what free chlorine you hold based on the CYA you actually run, and check it weekly rather than monthly. That is the whole content of salt water pool maintenance, and it is cheaper than the jugs of chlorine you just bought.

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