Why pools turn green, and how to fix it

The usual causes

Chlorine runs low

Sun, heat, heavy swimming and rain all use up chlorine. Once it drops too low, algae spores that blow in every day get a foothold.

pH drifts high

Chlorine works best with pH around 7.4 to 7.6. As pH climbs toward 8, the same amount of chlorine kills algae far more slowly.

Stabilizer is off

Cyanuric acid (stabilizer) shields chlorine from the sun. Too little, and the sun burns chlorine off in hours. Too much, and chlorine becomes sluggish.

Poor circulation or a dirty filter

If the pump doesn’t run long enough, or the filter is clogged, water sits still in corners and steps, and algae starts there.

Heavy rain

Summer downpours dilute chlorine and wash in leaves, dirt and fertilizer that feed algae.

How to clear a green pool

1. Test the water. Bring pH into range first, so the shock works.
2. Brush everything: walls, floor, steps and behind ladders, to break up the algae.
3. Shock the pool with enough chlorine for its size and how green it is.
4. Run the pump around the clock and clean or backwash the filter often. It will clog with dead algae.
5. Vacuum the settled debris once the water turns cloudy blue.
6. Retest and repeat until chlorine holds overnight and the water is clear.

Don’t swim in a green pool. If you can’t see the bottom, you can’t see someone in trouble.

When to call a pro

If the water is dark green or black, the bottom isn’t visible, or it hasn’t cleared after a few days, it usually needs a professional cleanup. Duff’s green-to-clean service starts at $350 and includes algae treatment, shock, deep cleaning, filter cleaning and rebalancing. After that, weekly service from $125 a month keeps it from happening again.

Questions