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Green Pool: Why It Happened & How to Fix It | Poolaroo

Quick Reference: A green pool = an algae bloom, and an algae bloom = free chlorine ran out (or wasn't high enough for your CYA level). The fix is sustained high chlorine plus continuous filtering, not a single dose. You almost never need to drain.

Green Pool: Why It Happened and How to Fix It

A green pool is almost always an algae bloom, and an algae bloom almost always means one thing: your free chlorine ran out, or never reached an effective level for your stabilizer. Algae are fast. A pool can go from clear to green in 24 hours during hot weather. The good news is that a green pool is one of the most fixable problems in pool care, and you almost never need to drain it.

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Step 1: Test, don't guess

Before adding anything, test your water, specifically free chlorine (FC) and cyanuric acid (CYA, also called stabilizer). Green water usually shows a free chlorine reading at or near zero. CYA matters because it determines how much chlorine you actually need: the higher your CYA, the higher the free chlorine level required to kill algae. This relationship is the foundation of the SLAM (Shock Level And Maintain) method used to clear algae.

Step 2: Read the shade of green

The color is a rough clue to how bad the bloom is, not a precise diagnosis, since metals and pollen can also tint water green:

  • Light teal / cloudy green: Early bloom. Often clears within a day or two of proper treatment.
  • Solid green: Established bloom. Plan on several days of treatment and filtering.
  • Dark green / swamp: Heavy bloom. Clearable, but expect a multi-day process, heavy filtering, and possibly a partial drain if you can't see the bottom in the shallow end.

Step 3: The core fix

The fix for green water is sustained, high free chlorine. Not a single dose, but a chlorine level held high enough, long enough, to kill every bit of algae. The two things that defeat people are stopping too early and not running the filter enough.

  1. Balance pH first to around 7.2. Chlorine is more effective (faster-acting) at lower pH, and very high pH slows the kill rate. Setting pH first also matters because pH testing becomes unreliable once free chlorine is at shock level.
  2. Raise free chlorine to shock level based on your CYA, and hold it there, retesting and re-dosing throughout the day, not just once.
  3. Run the filter continuously (24/7) until the water clears. The chlorine kills the algae; the filter removes the dead algae.
  4. Brush the walls and floor daily to break up algae clinging to surfaces.
  5. Clean or backwash the filter as it loads up with dead algae and pressure rises. Backwash for sand/DE filters, rinse/clean for cartridge filters.
  6. Confirm it's truly clear before stopping. The water should be clear, free chlorine should hold steady, and an overnight chlorine loss test should show little to no drop. That tells you the algae is actually dead, not just hiding.

Is it actually algae? Two things that look green but aren't

  • Metals (copper): If your free chlorine is genuinely holding at target and brushing doesn't kick up a cloudy green "dust," the color may be dissolved copper rather than algae. A sequestrant helps keep metals in solution and prevents staining, but it doesn't remove the copper, and you should keep free chlorine in range for your CYA while you sort out the cause. Don't stop chlorinating on the assumption of metals unless you've confirmed it.
  • Pollen and debris: Surface film, especially in spring, can look green but brushes and skims away and doesn't cloud the whole pool.

What about algaecide?

Algaecide is a supporting player, not the cure for an active green pool. Chlorine kills the algae; algaecide is better suited to prevention. Don't reach for it as your primary fix for an existing bloom.

Pro tip: The single biggest reason green-pool cleanups fail is quitting early. The water often looks "good enough" a day before the algae is actually dead. Hold shock level until free chlorine stops dropping overnight.

Special cases

  • Yellow-green that resists treatment? You may have mustard (yellow) algae, which is more stubborn than common green algae and needs an adapted protocol with higher chlorine targets and thorough cleaning of accessories.
  • Persistent re-greening? High phosphate levels feed algae and can make a pool harder to keep clear once chlorine slips. Worth investigating if you fight algae repeatedly. (Adequate chlorine for your CYA prevents algae regardless of phosphate level.)
  • Hot tubs/spas: Green spa water is rare and is treated differently due to the small water volume and high temperature. Shock with a spa-appropriate product and, if needed, drain and refill.

When is it time to drain?

Rarely. Consider a partial drain and refill mainly when very high CYA would force you to use an impractical amount of chlorine, or when debris and visibility are a genuine safety issue and you can't remove the solids effectively. In most cases, chlorine and patience win.