Why is my pool green?

Rain does three things to a pool at once: it dilutes the chlorine, it knocks the pH and alkalinity around, and it washes in everything from the yard. Leaves, pollen, grass clippings and fertilizer all end up in the water, and all of it eats chlorine or feeds algae.

Runoff carrying clippings and fertilizer is a well known problem for Florida waterways, and UF/IFAS explains where it goes. A pool next to a freshly fertilized lawn gets its share. In summer, with a storm most afternoons and the sun burning chlorine off in between, a pool can go from clear to green in a couple of days.

If it is still cloudy green, the shock was not strong enough to finish the job. If it turned green but clear, it is probably metal, not algae.

• Cloudy green after a shock: not enough chlorine for the amount of algae, pH too high for the chlorine to work, or so much stabilizer that the chlorine is held back. Fix the pH, shock harder after dark and keep the filter running.
• Clear green or teal right after a shock: chlorine oxidizing copper or iron in the water. This is common on well water. More shock makes it worse. See how to remove well water stains.

Because the test is not telling you everything. A pool can read fine and still lose to algae for a few reasons:

• The test is reading total chlorine, not free chlorine. Only free chlorine kills anything.
• Strips or old reagents. The CDC recommends a DPD test kit for chlorine and pH, and strips that have sat in a hot garage drift badly.
• Too much stabilizer. Cyanuric acid protects chlorine from the sun, but the more there is, the more chlorine it takes to do the same job. The CDC minimum goes from 1 ppm to 2 ppm once stabilizer is in the water, and pools that use stabilized tablets all year often sit far above that.
• Circulation. A dirty filter, a pump that does not run long enough, or a dead corner the returns never reach gives algae somewhere to start. See pool pump run time in Florida.

A light or cloudy green tint is usually algae just getting started, which means the chlorine is losing but has not lost yet. Shock it that evening and it usually clears in a day or two. If the water is light green but perfectly clear and the chlorine reads fine, suspect metal instead, usually copper, and see the section on green after shocking above.

Phosphates do not turn a pool green on their own, but they feed algae, so a pool full of them goes green faster the moment chlorine slips. They come in with rain runoff, fertilizer, leaves and some fill water.

A phosphate remover helps a pool that keeps going green, but chlorine is still the real defense. More in do phosphates matter in a pool. A pool with steady chlorine stays clear even with some phosphate in it.

No. Algae itself is not the main worry. Green water means the chlorine is losing, so it is not keeping up with germs either, and cloudy water hides the bottom. If you cannot see the main drain, nobody swims, because you cannot see someone in trouble underwater.

In summer, two or three days. A pool that misses a week of service in July, with storms most afternoons, can turn from clear to green between visits. In winter it is slower, but it still happens.

That speed is the whole case for weekly pool service here. A week is about as long as a Florida pool can be left alone in summer before the chemistry runs out.

Lower the pH to about 7.2, shock hard after dark, brush everything and run the filter around the clock. Most green pools clear in two to four days.

The full step by step, and the point where you should stop pouring chemicals in, is in how to get algae out of a pool. If you would rather hand the whole job off, a green pool cleanup gets it back to clear.