Why Your Pool Water Turns Green — And How Fast You Can Fix It

If you've walked out to your backyard and found your once-blue pool looking like a swamp, you're not alone. Green pool water is one of the most common calls we get in the Houston area, especially once the weather heats up. The good news: it's almost always fixable, and usually faster than people expect.

What actually causes green water

In nearly every case, green water means algae. Algae spores are airborne and waterborne — they blow in on the wind, hitch a ride on pool toys, or arrive on a swimsuit from another pool or lake. They're in the water constantly. What changes is whether your pool's sanitizer level is strong enough to keep them from taking hold.

A few conditions make algae blooms far more likely:

• Low or inconsistent chlorine. Even a day or two of low sanitizer during a heatwave is enough for algae to get a foothold.
• Poor circulation. Dead spots near steps, ladders, and behind fittings give algae a place to settle in undisturbed.
• High phosphate levels. Phosphates come from fertilizer runoff, leaves, and even some sunscreens, and they act like fertilizer for algae too.
• Skipped brushing. Algae anchors itself to walls and floors. If it isn't physically knocked loose, chemicals alone take much longer to work.

How fast can it actually be fixed?

A mild, early-stage green tint can often be cleared in 24 to 48 hours with a proper shock treatment, brushing, and non-stop filtration. A deep, pea-soup-green pool that's been neglected for weeks may take three to five days, sometimes with a second shock treatment partway through.

The steps we follow on a green-to-clean job are the same every time:

1. Test and balance pH and alkalinity first — chlorine can't do its job properly if the water chemistry is off.
2. Brush every surface thoroughly to break up algae clinging to walls, steps, and the floor.
3. Shock the pool with a strong dose of chlorine, calculated for the pool's volume and how green the water actually is.
4. Run the filter continuously (not on a timer) until the water clears.
5. Vacuum out the dead algae once it settles, and clean or backwash the filter, since a clogged filter is a common reason algae comes right back.

Keeping it from coming back

Once a pool is clear again, the goal is to stop the cycle from restarting. Weekly professional service exists largely for this reason — regular testing catches a sanitizer dip before it turns into a bloom, and regular brushing keeps algae from ever getting comfortable. Pools serviced consistently through the Houston summer heat rarely see a green-water problem at all.

If your pool has turned green more than once this season, it's usually a sign of an underlying issue — a pump that isn't circulating well, a filter that needs attention, or a sanitizer routine that isn't quite matched to your pool's size and sun exposure. That's worth a second look rather than just repeating the same shock treatment each time.