Cloudy Pool Water: Causes and How to Fix It

The quick answer: cloudy water comes from one of five things:

1. Early algae, because chlorine is too low for your CYA level.
2. Scaling, from high pH, alkalinity and calcium together.
3. Weak filtration.
4. Dead algae and debris after a shock.
5. Fine debris or particles from rain, pollen or a fresh plaster finish.

Test FC, CYA, pH, TA and calcium hardness, then run the overnight chlorine loss test. Those results tell you which cause you have, and the fix follows. Enter your readings below to see where your water stands.

Step 1: Rule out early algae (the most common cause)

Algae turns pools cloudy before it turns them green. Two tests catch it:

• FC vs CYA. Is your free chlorine at or above 7.5% of your cyanuric acid? With CYA at 60, that means at least 4.5 ppm. "3 ppm with 70 CYA" is a classic cloudy pool. Check the FC/CYA chart.
• Overnight chlorine loss test (OCLT):
  1. Test FC after sunset and write it down.
  2. Test again before sunrise. Add no chemicals in between, and keep the pump running as normal.
  3. A loss of more than 1 ppm means something is eating chlorine in the dark, almost always algae.

If either test fails, the fix is a SLAM. Raise FC to 40% of CYA and hold it until the water is clear, combined chlorine is 0.5 ppm or less, and the pool passes the OCLT. See how much shock to add.

Also check combined chlorine (CC). CC above 0.5 ppm means there are contaminants that chlorine hasn't finished breaking down. That's another sign to SLAM.

Step 2: Check for scaling (calcium carbonate haze)

If chlorine is fine and the OCLT passes, look at water balance. When pH, total alkalinity and calcium hardness are all high, especially in warm water, calcium carbonate comes out of solution. It shows up as a fine white or bluish haze, rough surfaces or a white crust at the waterline.

Classic triggers:

• Adding cal-hypo to water that's already hard and high-pH.
• Using soda ash to raise pH and overshooting.
• New plaster raising pH.
• Heating the pool. Warmer water scales more easily.

Check: calculate your CSI (Calcite Saturation Index) with the CSI calculator. Positive values above about +0.3 point to scaling.

Fix:

• Lower pH to about 7.2–7.4 with acid, and lower TA if it's high. See pool alkalinity too high.
• The haze usually redissolves or filters out within a few days.
• If calcium hardness is very high, the only way to lower it is partial drain-and-refill with softer water.

Step 3: Check filtration

Chemistry can be perfect while the filter is losing.

• Run time. Turn the pump up to 24/7 until the water is clear. Short timer schedules leave fine particles in suspension.
• Pressure. Note the clean-filter pressure. If it's more than 8–10 psi above clean, clean or backwash.
  • Sand: backwash. If the sand is many years old or clumped (channeling), it may need replacing.
  • Cartridge: hose it off pleat by pleat, and soak it in a filter cleaner if it's greasy. Replace it if it's torn or won't come clean.
  • DE: backwash and recharge with the right amount of DE. Grids can tear and let DE back into the pool (you'll see fine white dust).
• Circulation. Point returns to create a gentle circular flow. Brush the dead spots.

Sand filters are slow to polish very fine particles. A small amount of DE added through the skimmer (a common trick for sand filters, in very small amounts) or a clarifier can speed things up. They help; they don't replace fixing the cause.

Step 4: Cloudy after shock?

This is normal for a day or three. The chlorine has killed algae and broken down contaminants, and now the filter has to remove the remains.

• Keep the filter running 24/7 and clean it as pressure rises.
• If you were SLAMming, keep holding the shock level until the OCLT passes. Cloudiness is often the last stage of algae dying.
• If you shocked with cal-hypo, the cloudiness may be calcium carbonate. That's especially likely if pH and calcium were high. Bring pH down and check CSI.

Step 5: Cloudy after rain, pollen or a pool party

Rain dilutes chlorine and washes in dirt, pollen and phosphates. Heavy swimmer use adds sunscreen and body oils.

• Test FC and top it up to your target. After big storms, many pools need extra.
• Skim and brush; run the filter longer.
• Check pH. Rain is usually slightly acidic, but runoff from hardscape can push it either way.

Safety: can you swim in it?

A good rule: if you can't clearly see the main drain from the deck, keep everyone out. Cloudy water hides swimmers in trouble. And if the cloudiness is algae, the water isn't sanitized properly.

If you shocked to clear it: 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.

A quick diagnostic table

Track it with Pool Calc

Pool Calc flags low FC for your CYA before the water turns. It records your overnight-loss tests and shows CSI on every result, so you can see scaling coming. Every test goes into your log, so the cause is easy to spot next time.

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

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.