Green Water & Algae

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

• How to clear a green pool fast - the ordered fix with a dose table
• Best pool shock - which chemistry, and how much of it
• Best pool algaecide - prevention, and the one case where it is a rescue product
• Identify your algae - green, mustard and black behave completely differently
• Water bugs in a pool - what they eat, and why that is the fix
• Green water that is not algae - copper, iron and the pools that shocking makes worse
• Liquid chlorine vs shock - the same job, priced and dosed differently
• Cal-hypo vs dichlor vs trichlor - what each one adds besides chlorine
• Green pool after rain - dilution, phosphate and the 48-hour window
• How to get rid of black algae - the one algae chlorine cannot reach
• How to get rid of mustard algae - and why it keeps coming back
• Best pool vacuum for algae - where the dead algae actually ends up
• Best pool brush - bristle material by pool surface, and why steel cuts a liner
• Best pool metal remover - sequestrant or stain lifter, and the order to use them