Why Is My Pool Green? (And How to Fix It)

The Three Main Causes

• Low chlorine. This is the cause about 80% of the time. If your free chlorine drops below 1 ppm, algae starts growing immediately. Heavy rain, high bather load, hot weather, and a tired stabilizer level can all push chlorine down faster than you'd expect.

• Algae bloom after a storm. Florida storms dump a lot of water, debris, and contamination into your pool in a short time. The dilution drops your chemical levels and the debris feeds algae. After a major storm, a pool that was crystal clear yesterday can be cloudy or green by the next afternoon.

• Phosphates and nutrients. Fertilizer runoff, leaves, and organic debris feed algae. Even with normal chlorine levels, a pool with high phosphates can stay green or cloudy because the algae has a constant food source.

Quick DIY First Aid

If you want to try fixing it yourself before calling a pro, start here:

1. Test the water. Get an accurate read on free chlorine, pH, and alkalinity. If you don't have a reliable test kit, take a sample to a local pool supply store, as most will test it free.

2. Brush the pool. Algae clings to walls, steps, and the floor. Brushing breaks it loose so the chemicals can actually kill it. This step gets skipped a lot and it's why a lot of DIY shock treatments don't work.

3. Shock the pool. Add a heavy dose of chlorine (usually 2–3 times the normal amount). Calcium hypochlorite or liquid chlorine both work. Run the pump 24 hours.

4. Check the filter. A green pool means your filter is about to be loaded with dead algae. Backwash a sand or DE filter, or rinse a cartridge filter, after the algae starts to die off.

5. Retest and rebalance. Once the water clears, bring chlorine back to a normal level (1–3 ppm) and check pH and alkalinity.

This sequence handles most mild green pools in 2–4 days.

When to Call a Pro

Some green pools won't come back with a DIY shock. Call a pro if:

The pool is dark green or black-green. That's a heavy bloom and usually needs a multi-day chemical treatment plus a real cleaning to stop it from coming back.

You can't see the bottom. If visibility is gone, there's serious organic loading and the filter is probably overwhelmed. A pro will know whether to drain partially, do a flocculant treatment, or run extended filtration.

You shocked it and nothing happened. If chlorine isn't holding, you've got a chlorine demand problem, as something in the water is eating the chlorine as fast as you add it. This needs diagnosis, not more shock.

There's mustard or black algae. These are different from common green algae and require specific chemical treatments. Most homeowners don't have the right products on hand.

Equipment is acting up. A green pool plus a struggling pump or clogged filter is a much bigger job, and pushing failing equipment too hard during recovery can break it.

How to Stop It From Happening Again

Most green pools are preventable with consistent weekly maintenance. The two big things are keeping free chlorine in the 1–3 ppm range at all times and keeping cyanuric acid (stabilizer) in spec: too low and the sun burns off chlorine in hours, too high and chlorine becomes ineffective.

Brushing and skimming weekly prevents debris from feeding algae. After major storms, a quick chemistry check the next morning catches problems before they bloom.

If your pool keeps turning green and you're tired of fighting it, weekly professional service is usually cheaper in the long run than the cost of repeated emergency fixes, replacement chemicals, and equipment wear.

Need Help With a Green Pool?

Falk Brothers Pool Care services Lake Mary, Longwood, Sanford, and the surrounding Seminole County area. We can usually have a tech out within a few days for a free on-site assessment, including green pool recovery quotes. Family owned, 40+ years in the Central Florida pool industry.