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Skim&Soak

Green Water & Algae

Green Water & Algae

Green is not a mystery. It is arithmetic that got away from you, and the arithmetic is how you get it back.

By Scooter M.Last reviewed September 2, 2026How we work the numbers out
A swimming pool with green algae-colored water and a visible waterline

Start here if the pool is green right now

Read How to clear a green pool fast. It is the ordered fix, with a dose table for your pool volume and the arithmetic printed beside it. Everything else in this section is context you can read once the water is moving in the right direction.

If the water is green but you cannot see any algae on the walls, stop before you buy shock and read Green water that is not algae first. Roughly one green pool in five is a metals problem, and shocking a metals problem makes it worse rather than better.

Why pools go green, in one paragraph

Chlorine is consumed continuously by sunlight, by organic matter and by anything living in the water. Algae spores arrive constantly on wind, rain and swimsuits. As long as the free chlorine residual stays ahead of that demand, nothing visible happens. The moment demand exceeds supply for long enough - a hot week, a storm, a failed pump, a holiday - the spores get a foothold, and algae reproduces fast enough that the pool goes from clear to green in about thirty-six hours.

That is why the fix is a large, deliberate chlorine addition rather than a gradual one. You are not topping the residual back up; you are overwhelming a population that is currently winning.

The four things that decide how long this takes

  • **Your pool volume.** Every dose on this site is per gallon, because a dose that is right for 10,000 gallons is 50% short in a 15,000-gallon pool. If you do not know your volume, work it out here before you buy anything.
  • **Your cyanuric acid level.** Stabilizer protects chlorine from sunlight, and above roughly 50 ppm it also starts blunting it. A pool at 100 ppm CYA needs far more chlorine to reach the same effective sanitizing power. This is the single most common reason a shock dose 'does not work'.
  • **Whether you brushed.** Algae on a wall is physically shielded from the water around it. Brushing first is what lets the chlorine reach the organism instead of the film over it.
  • **Filter runtime.** Killing algae turns it gray; removing it is the filter's job. A pool that cleared to gray and stopped is a filtration problem, not a chemistry one - see cloudy water after shock.

What we will not tell you

We will not tell you that one pound of shock clears a green pool. That number is only true for a 10,000-gallon pool, a 68% cal-hypo product, and a bloom that has barely started. Change any of those three and it is wrong, sometimes by a factor of four. Every dose table on this site states the volume it assumes and the product percentage it assumes, and shows you the formula so you can redo it for yours.

Everything in this section

Every page in this section

Close-up of dark algae growth on a pool tile surface underwater

How to Identify Pool Algae

Where it grows and whether it brushes off are the two tests that separate green, mustard and black algae.

Discoloured swimming pool water with clean surfaces and no visible algae growth

Green Water That Is Not Algae

Copper and iron turn water green when they oxidize. The bucket test tells you which problem you have in five minutes.

Rain falling onto the surface of a swimming pool during a storm

Green Pool After Rain

Dilution, phosphate and a pH shift arrive together after rain. Test within 24 hours and the bloom never starts.

Questions people actually ask

How long does it take to clear a green pool?
A light bloom - water still translucent, floor visible - typically clears in 24 to 48 hours with continuous filtration. A pool you cannot see the first step through takes two to five days, most of which is filtration rather than chemistry. Anything longer usually means the filter is undersized, dirty, or the pump is not running around the clock.
Can I swim in a green pool?
No. Green means chlorine has been absent long enough for something to grow, and whatever else is in the water has had the same opportunity. The algae itself is not the hazard; the bacteria that share its conditions are.
Will shock alone clear it, or do I need algaecide too?
Shock kills the bloom. Algaecide prevents the next one. Adding a polyquat algaecide during a heavy shock mostly wastes it, because the chlorine consumes it - dose it after the water has cleared and chlorine is back to normal levels. See best pool algaecide.
Why did my pool go green again a week later?
Almost always one of three things: cyanuric acid high enough to blunt your chlorine, phosphate feeding a fresh bloom, or a filter that never got cleaned after the first one. Test cyanuric acid first, since it is the cheapest to check and the most likely.

Sources

Every figure on this page traces to one of these. Where a number could not be sourced, the page says so rather than estimating.

Why there is nothing to buy here: This is the section's front door rather than a recommendation. Each page below carries the products that suit its specific problem, at the point in the fix where you actually reach for them.