Green Pool Water: How to Fix It

The quick answer: a green pool means algae is growing faster than your chlorine can kill it. The reliable fix is to SLAM it:

1. Raise free chlorine (FC) to 40% of your cyanuric acid (CYA).
2. Hold it there, re-dosing several times a day, while you brush and filter.
3. Stop only when the water is clear, combined chlorine is 0.5 ppm or less, and the pool loses 1 ppm or less of FC overnight.

The calculator gives your first dose.

Example: a 20,000 gallon pool with CYA at 40 and FC at 0.

• Shock level: 16 ppm.
• First dose: about 3.2 gal of 10% liquid chlorine.
• Expect to add a lot more over the next few days. A green pool consumes chlorine fast until the algae is gone.

Why pools turn green

Almost always, it's one of these:

1. FC was too low for the CYA level. The most common case. Many owners keep "1–3 ppm" as advised, but with 60–80 ppm of CYA the minimum is 4.5 to 6 ppm, so that isn't enough to stop algae. See the FC/CYA chart.
2. Chlorine ran out: a heat wave, a party, a missed weekend of dosing, or a broken salt cell.
3. CYA built up from trichlor tablets or dichlor until normal chlorine levels stopped working.
4. The pool was opened after winter with little or no chlorine.
5. Poor circulation: dead spots, a pump on a short timer, or a clogged filter.

Phosphates, "chlorine lock" and pH are rarely the real cause. Algae grows when active chlorine is too low. Fix that and the pool clears.

Before you start: three tests

1. CYA. This sets your shock level. In green water, the CYA test can be hard to read. If needed, get a store test, or dilute the sample. If CYA is above about 80–90 ppm, consider replacing part of the water first. Otherwise the shock level becomes impractically high and expensive.
2. pH. Bring it to about 7.2–7.4 if it's high. It's easier to manage during the SLAM. Add acid, wait 30 minutes, then add chlorine. Never add them together.
3. Volume. If your gallons are a guess, every dose is a guess. Use the pool volume calculator.

Step by step

1. Clean out the big stuff. Scoop out leaves and debris. Organic matter consumes chlorine you need for algae.

2. Get the filter ready.

• Clean or backwash it.
• Run the pump 24/7 until you're finished.
• Sand and DE filters can be backwashed whenever pressure rises 8–10 psi above clean. Cartridges need hosing off, often daily at first.

3. Raise FC to the shock level.

• Use liquid chlorine or plain unscented bleach. Avoid dichlor and trichlor, which add CYA and raise the target as you go.
• Cal-hypo works if your calcium hardness is low.
• Pour it around the edges with the pump on, ideally at dusk.

4. Brush everything, walls, floor, steps and behind ladders, at least once a day. Algae hides in a protective film; brushing exposes it.

5. Test and top up.

• Test FC two to four times a day, and dose back up to the shock level each time.
• On day one, a green pool may lose most of the chlorine in a few hours. Keep going.
• The shock calculator gives each top-up amount.

6. Watch it change. Green turns to blue-grey, then cloudy white, then clear. The cloudy stage is dead algae, which your filter has to remove. Keep holding the shock level through it.

7. Run the overnight loss test.

• When the water looks clear, test FC after sunset, then again before sunrise.
• A loss of 1 ppm or less means the algae is gone.
• More than 1 ppm means it's still there. Keep going.

8. Confirm combined chlorine (CC) is 0.5 ppm or less. Then stop adding chlorine, and let FC drift down to your normal target.

How long will it take?

The biggest variables are how consistently you hold the shock level overnight, and how well your filter copes. A cartridge filter may need cleaning several times. DE needs recharging after each backwash.

What about algaecide, flocculant and phosphate remover?

• Algaecide: not needed to clear a pool. A non-foaming polyquat 60 algaecide can help as prevention afterwards. Avoid copper-based algaecides; copper can stain plaster and turn hair green.
• Flocculant: drops cloudiness to the floor so you can vacuum it to waste. It's useful if your filter is weak, but you lose water and it adds work. It doesn't kill anything.
• Phosphate remover: optional. With proper chlorine, algae can't grow regardless of phosphate levels.
• "Green-to-clean" kits usually combine shock and algaecide in amounts sized for an average pool. They often fall short of the shock level for your CYA.

After the pool is clear

1. Keep FC above the minimum for your CYA every day.
2. Log it. Overnight loss creeping up is the earliest warning, days before you see green.
3. Check CYA every month, especially if you use tablets.
4. Clean the filter once more after the SLAM to clear out the dead algae.

Clear it with Pool Calc

Start a SLAM in Pool Calc and it:

• sets the target from your CYA;
• calculates every top-up for your product;
• reminds you to retest;
• tells you when you've passed the CC and overnight-loss checks.

The log shows exactly how much chlorine the clean-up used, which is handy for preventing the next one.

One pool? Every feature, free. No account, no ads, and iCloud backup is included.

Before anyone swims

Keep swimmers out until the water is clear (you can see the main drain) and the SLAM is over. Swim when free chlorine is back within your normal range and at or below the re-entry level on your chlorine label (many labels say 4 ppm). Never swim during a SLAM or with free chlorine above 10 ppm. Your product label always comes first.

Safety

• Keep chemicals out of reach of children and pets, and keep them out of the water until it is back in range.
• Never mix chemicals. Never add acid and chlorine together: mixing them releases toxic chlorine gas.
• Add chemicals to water, never water to chemicals.
• Store chemicals sealed, cool and dry, apart from each other.
• Read and follow the product label. It always comes first.