If you have a backyard watering hole, you’ve experienced this at one time: an ugly, green pool. Before you panic, read our guide — we’ll show you how to avoid turning your pool into a swamp.
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Have a Green Pool?
Every pool owner deals with it on occasion: The dreaded green pool. If you know how to clean a green pool, you can keep your pool crystal-clear and inviting.
Following the steps in this guide will turn your green pool back into a backyard paradise. No matter how attentive you are to your pool, sometimes it ends up looking more like split-pea soup than a blue lagoon.
This probably isn’t how you imagined life as a pool owner would be! Don’t worry. Cleaning a green pool is actually pretty simple once you know and understand what the different pool chemicals do.
What We’ll Cover
In our guide, we cover the different things that can cause your pool water to turn green, how to clean a green pool, a few helpful tips, and common pool-cleaning myths that can actually make the problem worse.
There’s no need to miss out on another pool day when you can restore your pool to its former glory within a few hours.
Every pool owner needs to know what’s causing their pool to turn green so they can take steps to combat it and prevent it from happening in the future.
We’ve researched and collected tips from pool and spa experts around the country to create this handy guide. Up first: What’s causing your pool to turn green.
Why Your Pool Is Green
Pool water is kept sanitized and clean with the help of a balanced pH level and chemicals like chlorine. As long as a sanitizing agent is added in regular intervals, it helps the water stay clear by killing bacteria and algae.
In addition to a sanitizing chemical like chlorine, your pool also requires balanced levels of other chemicals to stay clear. You’ll need a pool water test kit to check these levels regularly.
They’re available in liquid kits, strip kits, and digital kits. If any of these pool chemical levels are off by much, your pool water will suffer as a result.
Unbalanced chemicals can also cause rashes, skin infections, and injuries from slick, slimy algae growth.
Other Chemical Considerations
If your pool chemical levels are kept relatively constant (it’s recommended you test at least every morning), it prevents bacteria and algae from growing in the pool water.
If the chlorine levels dip below the recommended amount, or if your other pool levels aren’t balanced, the door opens for these organisms to start growing in the water.
And if they stay low for long, bacteria and algae begin to flourish. You’ll know they’re flourishing when you see the water start to turn green (it can also turn yellow, blue, or black) or cloudy.
Your pool can turn green if your pool filter is clogged and dirty. A clogged filter will be so full of muck that it can’t filter out any more particles or waste from the water.
This can change the pool water’s pH and give algae and bacteria a foothold to start growing. It’s hard to believe, but even rain can cause your pool to turn green.
With the addition of fresh, unchlorinated water to the pool, all the chemical levels are thrown off balance. This can make conditions ripe for algae to grow and take over, turning the water green.
How to Clean a Green Pool
Now that you know what may be causing your pool water to turn green, let’s get to the important stuff: How to clean a green pool.
Cleaning a green pool means getting on a first-name basis with pool chemicals that sound complex and intimidating at first, but are simple chemicals you might use in your home as well.
1. Test the Water
If you don’t already have one, you need to purchase a pool water test kit. The most accurate kits are those that come with tiny vials of multicolored liquids.
With these kits, you collect some of the pool water and follow the instructions to drop each testing chemical into it. The color the pool water turns indicates the level of the chemical you’re testing for.
A healthy, clear pool will has the following levels:
- pH: 7.4 to 7.6
- Chlorine (sanitizer): 1 to 3ppm
- Total Alkalinity: 80 to 140ppm
- Calcium Hardness: 200 to 400
- Cyanuric Acid (stabilizer): 25 to 50ppm
If your pool water is green, you’ll see that one or more of these levels are completely off when you test. Make note of which levels need to be adjusted before you head to the store.
If you’re new to pool care, you might consider taking a photo of each test result color and bring your test kit to the pool and spa store for help.
Your pool supply store will be able to help you calculate your pool’s volume as well, so you’ll know how much of each chemical to buy and add to your pool.
2. Buy and Add the Right Chemicals
The next step to clearing a green pool is buying and adding the right chemicals to correct the balance. Pool chemicals are usually labeled for what they accomplish, not what the actual chemical is.
Always treat alkalinity before treating pH levels. Sometimes adjusting alkalinity will automatically buffer the pH and may not require any additional chemicals.
You can use sodium bicarbonate (baking soda) to increase alkalinity and sodium bisulfate (dry acid) to decrease it. If you added chemicals to adjust alkalinity and the pH levels are still off 6 hours later, you can adjust the pH.
Increase Pool PH
If your pH is low (too acidic – below 7.4), you need a pH increaser (like pH Plus or pH Increaser). High pH levels (too alkaline – above 7.6) require a pH decreaser.
If your Cyanuric Acid (CYA) level is low, you’ll need a stabilizer (like Conditioner or Instant Pool Water Conditioner) to increase the level.
Most pool-specific chlorine has CYA added, so the level should stay constant as long as the water is properly chlorinated. Calcium hardness levels are important to maintain to keep your pool equipment and interior from being damaged.
Adding Calcium Carbonate
Add calcium carbonate to raise the calcium hardness level. If it’s too high, above 400, you’ll need to partially drain the pool then refill it.
Pool shock (usually calcium hypochlorite, or Cal Hypo) is another important component in balancing your pool’s chemical levels. Shock provides Free Chlorine (FC) to your pool without the stabilizer CYA.
Free Chlorine is chlorine that is available and unused in the pool water. Because it’s not dissolved in the water, this chlorine is “free” to kill algae and bacteria.
Shock the Water
Pool shock causes a temporary surge in the FC levels in pool water, which shocks and kills the algae and bacteria. You should use pool shock every week to help prevent algae blooms.
Algaecide is another possibility, but you probably won’t need it for a green pool. It’s more useful for the rarer and harder-to-kill types of algae growth like mustard and black algae.
The other chemicals listed are more than enough to take care of the common green algae. Consider stocking up on pool chemicals like these to cut down on trips to the pool supply store.
They’ll come in handy before a rainstorm or after a pool party, when the pool chemical levels are most unbalanced.
3. Loosen Algae and Vacuum the Pool
Use a pool brush to scrub the sides of your pool and the surface of stairs. This loosens any algae or bacterial film clinging to the sides and expose it to the chemicals you’ve added.
Once you’ve treated the pool with the correct amount of chemicals for your pool volume, the dead algae and bacteria will begin to fall to the bottom of the pool. From here, they can easily be vacuumed up.
Vacuum the pool by hand or using an automatic pool vacuum to ensure the dead organisms and any sediment are removed from the bottom. Run the vacuum as often as you can for a day or two.
This also prevents your filter from becoming clogged with algae and bacterial debris. There will be a lot more sediment getting filtered out than usual after a major chemical treatment.
4. Use an OCLT Test
After adding chemicals to balance the pool’s chemical levels, using a healthy dose of shock, brushing the sides, and vacuuming the bottom for a day or two, the water should look clear again without a trace of green.
But is the algae really gone? There’s only one way to know without waiting for the water to turn green again. Use an OCLT (Overnight Free-Chlorine Loss Test) to find out.
This test will tell you whether algae is present in the pool because it measures how much chlorine is lost overnight. The only two things that use chlorine up (resulting in chlorine loss) are sunlight and killing organic matter like algae.
By doing this test overnight, you’ll know sunlight is not the cause if there’s chlorine loss. That would point to algae being the culprit. If that’s the case, do another pool water test to see what may be excessive or lacking in your pool water.
You can shock the pool again to kill off any remaining algae if all the chemical levels are balanced. After two tests with no FC change, you’ll know you’ve officially killed off the algae causing your green pool.
Final Things to Consider When Cleaning a Green Pool
Now that you know how to clean a green pool, you’re equipped to be a more responsible pool owner. Regularly testing and adjusting your pool water’s chemical and pH levels will keep your pool clear, inviting, and safe to use all season long.
Before you dive in, just keep these final considerations and tips in mind.
- Don’t get into green pool water to clean it. Algae blooms can cause skin infections, irritation, and injuries due to the slippery coating it creates on the pool’s bottom and sides.
- If you’re expecting a good rain, add chemicals before it starts to rain. This will prevent the fresh water from diluting the chemicals in the pool.
- Don’t vacuum a green pool without using chemicals to treat the pool water levels. It will only remove some of the algae, and anything left will rebloom.
- Clean and change your pool filter regularly. Once it’s clogged and dirty, it can’t filter anything more. Everything it can’t catch will stay in your pool and encourage bacteria and algae growth.
- Don’t use bleach to chlorinate a pool. Household bleach doesn’t contain CYA, so it isn’t stabilized, and the chlorine will dissolve too fast to be effective.
That’s it – you’re ready to get your pool sparkling clean and properly maintain it! You may feel more like a chemist than a sunbather, but your pool is a big investment.
In the end, a clean, balanced pool is worth the small cost of daily water testing and regular maintenance.