Solved! Why Is My Pool Green?

The last thing you want to see as you head toward the pool for a refreshing dip is green pool water. If you’re wondering, “Why is my pool green?” learn about the causes and how to avoid them.

By Meghan Wentland | Updated Oct 19, 2021 4:45 PM

Why Is My Pool Green

Photo: depositphotos.com

Q: I got home from work last night and couldn’t wait to get into my pool to soak away the day, but when I got to the backyard, the pool water was green—not light green, either, but dark. I didn’t even get in! Why is my pool green?

A: It’s an unpleasant surprise to find your pool water green. You may wonder if it’s dangerous, or if you’ve done something wrong, or even if your jealous neighbors have pranked you. Most likely, the reason your pool water is green is a chemical imbalance. There are several reasons your swimming pool water might be green in both inground and above-ground pools, and the fix will be different depending on which kind of imbalance you have.

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Algae overgrowth is the most common cause of a green pool.

Algae is the primary reason for green water in your pool. Algae are normally kept in balance by water circulation, natural die-off in cool weather, and appropriate chemical balance. If the water circulation or chemicals are off, algae can bloom and cause a pool to be cloudy and green. While you’ll encounter naturally growing algae in ponds and streams, an overgrowth of algae in a swimming pool is more highly concentrated and can be more of a health hazard. Plus, its presence means there’s an imbalance that may also indicate dangerous bacteria growth. Don’t swim until the water is clear again.

Pollen and metals can also cause a pool to turn green. 

Pollen’s naturally greenish-yellow shade can contribute to a green pool, as can the oxidation of metal elements in the pool itself (including pump parts and ladders). Sometimes those metals can react with pool chemicals and release material into the water that throws off the chemical balance. To avoid too much pollen in the pool, use a pool cover when the pool is not in use. Have a pool professional check that your equipment is in good shape and isn’t deteriorating.

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The free chlorine levels might be low. 

The heady scent of chlorinated pool water can bring back memories of childhood summers, but in a well-balanced pool, you shouldn’t be able to smell it. Chlorine is used to purify water, including most municipal drinking water, because it will kill nearly any organism that could be harmful to humans. When the levels are properly balanced, chlorine will keep the algae at bay, but the water will slowly begin to turn green as the algae take over if there’s not enough. But be careful—adding too much chlorine in pool water can cause those metals to oxidize and turn the pool a different shade of green. If you’re struggling with balancing chlorine, you can consult a pool professional to discuss other options for sanitizing your pool.

Why Is Pool Water Green

Photo: depositphotos.com

The pH may be too high. 

pH is the measurement of the balance of how alkaline or acidic something is. The goal is to keep the pool water reasonably neutral, with a pH of around 7.3 to 7.6. If the pH becomes too high (over 7.8), it prevents the chlorine from doing its job. If you’re not checking the pH and it has risen too much, you could be adding the right amount of chlorine, but it can’t work correctly, and algae will begin to grow. In this case, you’ll need to add a pH reducer like sodium bisulfate or muriatic acid to bring the pH down to the correct level.

The filter might not be running long enough, or it needs to be replaced. 

Your pool’s filter has an important job: It sucks small debris and dirt out of the water and removes algae and pollen as the water is forced through (this is why you’ll notice that the filter is mostly green when you clean or change it). If you’re also asking yourself, “Why is my pool cloudy?” then the problem is probably the filter. You may not be running the filter long enough to clean the water well—in warm weather, the pool filter should be running for 8 hours a day, and if there are lots of people using it, potentially longer. If you’re running it for an appropriate length of time, the filter may need to be cleaned or changed. Cartridge filters should be cleaned every 2 to 6 weeks, diatomaceous earth (DE) filters should be backwashed every 1 to 3 months, and sand filters should be backwashed every 1 to 4 weeks. If this doesn’t correct the problem, the filter and pump may need to be replaced.

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Test the pool’s phosphate levels. 

Phosphates in the water can be a food source for algae, so if the level of phosphates in your pool water (usually included in a basic pool water chemistry test) is too high, your pool is essentially a buffet for algae. The answer to this is balancing the level of chlorine or another sanitizer. Some websites suggest aggressively scrubbing the water and walls or dumping heavy phosphate remover chemicals in your pool, but those chemicals won’t make the difference. Only balancing the chlorine levels will help.

Contact a pool care professional to ensure clear water. 

Why Is Pool Water Green

Photo: depositphotos.com

It’s reasonable to expect that you can maintain the chemical balance of a small pool on your own. Larger pools are more complicated, simply because more debris lands in them and there’s more to filter and clean. If you don’t know how to shock a pool to reset the balance, how much chlorine to add, or how to bring down a pH that’s too high, it’s a good idea to call a pool professional to reset the chemical levels for you. Or if you know how to perform those tasks but you’re having recurring pool issues, a pool pro might be able to give you more specialized care tips.

Many pool care companies provide seasonal contracts that reduce the cost of pool maintenance. You can decide to hand off the responsibility of pool care to someone else or just schedule visits for opening and closing the pool. While there’s a cost involved, there’s a decent chance you’ll spend a lot of money on chemicals you don’t need as you try to figure out why your pool is green, so hiring someone who really knows what they’re doing could be less expensive in the long run.

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