How to clear a green pool

Your chlorine target and the label's dose

The calculator above shows the minimum and everyday target free chlorine (FC) for your stabilizer (CYA). With CYA 40, the minimum is 3 ppm and the target is 3–5 ppm. A green pool is far below that. For the treatment dose itself, it points you to your product's label.

Why we don't print a treatment level: pool chlorine is a registered pesticide, and its label sets the amount for algae or shock treatment and when you can swim again. Some water-care methods hold chlorine for days at levels above the swim limit on many labels; we don't give those numbers. If your label lists an algae dose, use it, scaled to your gallons. Labels usually give an amount per 10,000 gallons, so a 15,000-gallon pool needs 1.5 times that amount.

Three numbers decide how the job goes. Pool volume scales every dose. CYA (cyanuric acid, the stabilizer) protects chlorine from sunlight but also holds most of it in reserve, so the more CYA you have, the harder algae is to clear at label levels. Product strength decides how much of your jug or bag the label amount is. If you only have a strip that reads CYA in wide steps (0, 30, 50, 100), pick the step your color matches. If CYA is off the chart, read the section on high stabilizer below before you buy anything.

Day 0: test, balance pH, brush, filter

Day 0 is setup. The goal is to prepare the pool so chlorine can work as well as possible.

1. Scoop out leaves and debris. Organic matter uses up chlorine that should go to the algae.
2. Test free chlorine, pH, total alkalinity, and CYA before you add anything.
3. Bring pH to 7.2–7.5 if it is higher. Chlorine works better at lower pH: the CDC model code lists roughly 50% of chlorine in its active form at pH 7.5 and about 20% at pH 8.0. Use the acid dose for your water, wait at least 30 minutes with the pump running (longer if the acid's label says so), then add chlorine.
4. Clean or backwash the filter and run the pump around the clock. Dead algae has to be filtered out, and a clogged filter is the most common reason a pool stays cloudy after it stops being green.
5. Brush walls, steps, and floor, then add your chlorine product at the amount its label gives for algae, scaled to your gallons. Add it the way the label says. Don't go above the label amount.
6. Add chlorine in the evening when you can. Sunlight breaks chlorine down, so a dose added at dusk works all night.

The next days: brush, filter, test, follow the label

This is the stage where most green-pool treatments fail. Algae uses chlorine fast, so a single dose can fade within hours and the algae recovers. What helps:

• test free chlorine at least twice a day, morning and evening;
• if your label allows a repeat treatment or lists a maintenance dose, follow it exactly; don't add more than the label allows;
• brush once or twice a day and clean the filter whenever pressure rises or flow drops;
• keep testing pH every day or two and keep it in range.

Expect the color to go from green to blue-gray to cloudy white before it clears. White haze is usually dead algae waiting for the filter. If the water hasn't improved after following the label for several days, check CYA and the filter, and consider calling a pool professional.

How to know it's done (overnight chlorine loss test)

Clear water alone does not mean you are done. Algae left in pipes, filters, and corners can still use up chlorine and bring the problem back a week later. Use three checks together:

1. Clear water. You can see the main drain at the deep end.
2. Combined chlorine (CC) 0.5 ppm or less. CC is total chlorine minus free chlorine. The combined chlorine calculator does the math.
3. Overnight chlorine loss of 1 ppm or less. Test FC after sunset, write it down, and test again before sunrise. With no sun on the water, a clean pool loses very little chlorine overnight. A bigger drop means something is still using chlorine, most often algae you can't see.

The overnight test was popularized by the Trouble Free Pool community; we could only confirm it through secondary sources. It works at any chlorine level, and it is a clear signal that the job is finished. When all three pass, go back to your everyday target (see pool chlorine levels).

Why shock bags fail (CYA)

A "one bag per 10,000 gallons" instruction has to be scaled to your pool. Here is how much one 1-lb bag actually raises free chlorine, from our engine's formulas (not a dose recommendation):

One bag in a 20,000-gallon pool adds about half as much as in a 10,000-gallon pool, so an unscaled bag can look like a shock that "didn't work."

The product matters too. Dichlor adds about 0.9 ppm CYA for every 1 ppm of chlorine (PHTA), and trichlor tablets add about 0.6 ppm. Each dose of stabilized chlorine raises CYA, and higher CYA makes algae harder to clear at label levels. Cal-hypo adds calcium instead, about 0.7 ppm of hardness per 1 ppm of chlorine. For a green pool, many owners use an unstabilized product labeled for pools, such as liquid chlorine or cal-hypo. Compare amounts on the liquid chlorine calculator.

When CYA is already high. Above about 50 ppm, even the everyday minimum for your CYA rises past the 4 ppm swim limit on many labels (see chlorine lock), and algae becomes hard to clear. Many CYA tests are unreliable above 100 ppm. Only water replacement lowers CYA, so a partial drain before you start is often the better first step. Vinyl-liner and above-ground pools should not be emptied completely: follow the maker's instructions. The CYA drain calculator works out how much to replace.

Algae vs metals: when green isn't algae

Not every green pool has algae. Yellow-brown dust on shady walls that brushes off easily is often mustard algae, and pink slime on fittings is a bacterium, not algae. Clues that point to metals instead:

• the water is clear but tinted green or teal, and walls feel smooth rather than slimy;
• the color appeared right after you added chlorine, especially with well water or after a copper-based algaecide;
• chlorine holds steady overnight while the color stays.

Chlorine oxidizes dissolved copper and iron, which can tint the water or leave stains. More chlorine will not fix that. See how to tell metal from algae and treat stains. If the water is both slimy and green, treat the algae first.

Is it safe to swim?

No, not in green water. There are two separate reasons. First, green or cloudy water hides swimmers: the CDC advises that the drain at the bottom of the deep end should be visible before anyone swims. Second, during treatment, chlorine can be above the swim limit on the label. Wait until the water is clear, the label's re-entry directions are met, and free chlorine is back at or below the swim limit on your chlorine label (4 ppm on many labels). The "Can I swim yet?" check compares your reading with your label.

FAQ

What causes a pool to turn green?

Algae, which grows when free chlorine drops below the minimum for your stabilizer (CYA). Common triggers are hot weather, heavy use, a storm, missed doses, a pump that stopped, or CYA that crept up over the season. If the water turned green right after you added chlorine, it may be metals instead: see "Algae vs metals" above.

What kills pool algae the fastest?

Chlorine at the amount your product's label gives for algae, together with brushing, a clean filter, and pH at 7.2–7.5 so chlorine works better. Steady effort matters more than a bigger dose: brush daily, keep the filter clean, and don't let chlorine run out between treatments.

How long does it take for a green pool to clear up?

There is no fixed number of days. It depends on how much algae there is, your filter, your stabilizer level, and how closely you follow the label and keep brushing. You are done when the water is clear, combined chlorine is 0.5 ppm or less, and free chlorine drops 1 ppm or less overnight.

Can you fix a green pool without draining?

Often yes. Draining part of the water is worth it mainly when CYA is high, because high stabilizer makes algae hard to clear at label levels. In that case, a partial drain to lower CYA comes first.

How do I turn a green pool back to blue?

Test, lower pH to 7.2–7.5, brush, clean the filter, and treat with a chlorine product labeled for algae control at its label amount for your gallons. Keep brushing and filtering, follow the label for any repeat dose, and stop when the three checks above pass. Then go back to your everyday chlorine target.

Sources

• CDC, Model Aquatic Health Code 2024, Annex (active chlorine vs pH, chlorine and CYA): cdc.gov (PDF)
• CDC telebriefing on public pool inspections, "Make sure the drain at the bottom of the deep end of the pool is visible": archive.cdc.gov
• U.S. EPA, examples of pool chlorine labels with a re-entry limit of 4 ppm and 3 ppm: EPA label (PDF), EPA label (PDF)
• PHTA fact sheet, Dichlor (CYA added per ppm chlorine): phta.org
• Secondary: overnight chlorine loss test as summarized from the Trouble Free Pool method: poolchemtracker.com