Why Is My Pool Green? Causes and How to Clean It

What’s Turning Your Pool Green?

So, you walked outside, looked at your pool, and thought—why does it look like a swamp? A green pool usually means algae have moved in, and they aren’t leaving without a fight. Let’s break down what might be behind the color change.

• Algae is the usual suspect: When chlorine levels drop, algae sneak in and spread fast. Warm, sunny days give algae the perfect environment to bloom.
• Low chlorine levels are often to blame: If your chlorine takes a dip—even for a day—your pool turns into an algae-friendly hangout. Sunlight, rain, and even a pool party can knock those levels down quickly.
• Poor filtration doesn’t help: If your filter’s clogged or your pump isn’t running long enough, your water won’t circulate properly. That gives algae more chances to settle in places like corners or steps.
• Chemical balance is key: When your pool’s pH or alkalinity is off, chlorine doesn’t work as it should. A high pH level, especially, weakens chlorine’s power to kill germs and algae.
• Phosphates feed algae: These come from fertilizers, leaves, and even city water. Algae thrive on them, and you may end up dealing with green water even if your chlorine is spot on.
• Don’t forget the weather: Hot days and heavy rain mess with your pool chemistry. Rainwater can dilute your chlorine, and the sun burns it off faster than you’d think.

How Green Is Too Green? Understanding Pool Color

Not all green pools are created equal. The shade of green tells you how far things have gone.

• Light green water: Usually means you’re catching the problem early. It’s still manageable with basic shock treatment and a few adjustments.
• Medium green water: Points to a moderate algae bloom. You’ll need to scrub, shock, and possibly add algaecide.
• Dark green or nearly black water: Is serious. At this point, it’s unsafe to swim in, and you’ll be looking at a heavy-duty cleaning process—possibly even professional help.

Either way, the deeper the green, the more attention your pool needs, and fast.

Before You Treat It: Testing the Water

Before throwing in chemicals, grab your test kit or some high-quality test strips. You want to check levels for:

• Free chlorine
• pH
• Total alkalinity
• Phosphates (if applicable)

This step matters because treating a pool without knowing what’s wrong is like guessing what’s in a recipe. Test first so you can dose everything correctly and avoid wasting time and money.

How to Clean a Green Pool Step by Step

Let’s walk through what it actually takes to fix green pool water. It’s a process, but it works.

Start With Cleaning Out Debris

• Clear the surface and bottom: Get the big stuff out of the way. Leaves, sticks, and other junk in the water add phosphates and make chlorine work harder. Skim everything you can and vacuum what’s settled on the bottom.

Balance the Chemistry

• Adjust pH and alkalinity: Check the pH and alkalinity. You want your pH between 7.2 and 7.6. Anything higher, and chlorine won’t do its job well. Use a pH reducer if needed and make sure alkalinity is somewhere between 80 and 120 ppm.

Shock It Hard

• Dump in the chlorine: You’ll need to shock the pool—like, really shock it. This means dumping in a big dose of chlorine, usually calcium hypochlorite or dichlor shock. The goal is to raise chlorine levels to at least 10 ppm. For darker water, you’ll need even more.
• Do it at night: Make sure to shock in the evening so sunlight doesn’t zap your chlorine before it starts working. Run your pump the entire time to keep the chemicals moving through the water.

Consider Algaecide and Clarifier

• Use after shock: If you’ve got a stubborn bloom, consider using an algaecide after you shock. Don’t add it before—chlorine needs to do its thing first.
• Add clarifier if needed: A clarifier can help too by clumping tiny particles together so your filter can trap them more easily.

Brush Every Surface

• Scrub every inch: Algae stick to pool walls like glue, so get in there with a brush. Use a stiff one for concrete and a soft one for vinyl. Hit every corner, step, and surface—don’t skip anything.

Let the Filter Run

• Run it nonstop: Your filter is your best friend now. Let it run non-stop for 24 to 48 hours. Clean or backwash it regularly during this time to keep it from getting overwhelmed by all the gunk it’s pulling out.

Vacuum the Rest

• Get what’s left: Once the water starts clearing up, it’s time to vacuum again. Manually this time. Don’t use your robotic vacuum yet—it might just stir everything up again. Focus on getting the dead algae off the bottom.

Test Again

• Make final checks: Now that your water is looking better, retest everything. Make final adjustments to chlorine, pH, and alkalinity before you let anyone jump in.

Keeping Your Pool From Going Green Again

Nobody wants to go through all that twice. Here’s how to stop green water from coming back.

• Test water weekly: Check your chlorine, pH, and alkalinity at least once a week. More often if it’s hot, rainy, or your pool’s getting a lot of use.
• Maintain chlorine levels: Aim for 2.0 to 4.0 ppm for free chlorine. Use stabilized chlorine tablets or granules and don’t let levels drop too low.
• Run the filter daily: Your filter should run 8 to 12 hours daily during swimming season. This keeps the water moving and algae from settling.
• Clean weekly: Brush the walls, vacuum the floor, and scoop out debris before it becomes a problem.
• Shock after heavy use or rain: Did it rain? Did ten kids spend all day splashing around? Time to shock the pool to bring chlorine back up and stop algae from taking hold.
• Use a pool cover: A pool cover keeps out leaves, bugs, and rain while reducing chlorine loss to the sun.
• Control phosphates: If you’ve got a recurring algae issue, use a phosphate remover once a month. This cuts off the algae’s food source and makes your sanitizer more effective.

When It’s Time to Call in a Pro

Sometimes, no matter what you do, the green just won’t go away. That’s when it’s time to bring in a pool expert.

• Persistent algae: If your pool stays green after multiple cleanings and shocks, something deeper could be wrong.
• Filtration issues: A pro can test your pump and filter setup to make sure everything is working as it should.
• Metal stains or black algae: These are more difficult to remove and usually require professional products and expertise.
• Chemical confusion: Not sure how to balance everything? Pool techs know how to get levels where they need to be.

Conclusion

A green pool isn’t just a cosmetic issue—it’s a warning sign that your water chemistry, filtration, or maintenance routine is out of balance. Fixing it takes some work, but with the right steps, you can get your pool looking clear and inviting again. From testing and shocking to scrubbing and running your filter non-stop, every action plays a role in kicking algae to the curb. And once you’ve restored it, stay on top of your routine to keep the green from creeping back.

Key takeaway: A green pool usually means algae have taken over because of low chlorine or bad filtration. The fix involves testing, shocking, scrubbing, and filtering like crazy. Keep it from coming back with regular care and consistent chemical balance.

FAQs

Can I still swim if my pool water is light green?

Swimming in green water isn’t a good idea, even if it looks mild. Algae and bacteria could still be floating around, causing skin and eye issues, especially for kids or sensitive swimmers.

Does green pool water always mean algae?

Most of the time it does, but not always. Sometimes metals like copper or iron in the water can cause a green tint. That’s why testing is important before treatment.

How much shock do I need for a green pool?

It depends on how green the water is. A light green pool might need one pound of shock per 10,000 gallons. If it’s dark green, you’ll need two or three times that.

Can rainwater turn my pool green?

Definitely. Rain can dilute your chlorine and introduce phosphates, both of which make it easier for algae to grow. Always test your pool after a storm.

Why is my pool still cloudy after shocking it?

That cloudiness is probably dead algae and debris floating around. Run your filter non-stop, add a clarifier if needed, and keep brushing and vacuuming until it clears up.