Skip to main content

How to Keep Algae out of Your Pool

Rob is a pool-service industry professional with over 20 years of experience.

There is hope—you CAN kill the algae in your pool! Read on to learn how.

There is hope—you CAN kill the algae in your pool! Read on to learn how.

How to Keep Algae Out of Your Pool

Algae is a common problem for pool owners. It's important to regularly take precautions to stop the growth of algae. It can be damaging to your health. The longer you put off doing something about your algae problem, the worse the problem will get, and the harder it will be to deal with.

If the water in your pool is too green, you may need to take immediate action to clean your pool. In severe cases, some people will need to have their pools drained and acid washed. However, in many cases, algae can be defeated or prevented.

Generally, if you can see at least six to eight inches below the surface of the water, most likely the pool can be treated chemically. Once we establish that the pool doesn't need to be drained and can be treated chemically, you should follow these steps.

Here's the algae troubleshooting steps you'll learn in this article:

  1. Test the Water
  2. Shock the Water
  3. Pump and Filter
  4. Brushing and Filtration

1. Test the Water

What does it mean if the pool is green? This means that there is, obviously, very little to no chlorine in the pool. So testing for chlorine is not very important, since you'll be shocking the pool anyway.

But pH plays a very important role. If the pH is very high, the shock will turn the pool cloudy. It will be cloudy anyway until it all filters out, but high pH will cause a VERY cloudy pool when using shock. I prefer to use a higher-end test kit, but the cheap test strips will, at least, give you an idea if your pH is high or low.

You want your pH to be low. If the pH reads high, use one gallon of muriatic acid, which should be enough. Don't worry about adding too much acid. Test the pH again after shocking and 4 hours of circulation.

What to Check for in Your Pool Water

  1. Check your chlorine levels. Chlorine levels should be between 2.0 and 5.0ppm (parts per million) If you've just treated your pool or turned over a green pool it may be higher than this. It will come down on its own. Most pool stores will do this for you at no charge.
  2. Check your pH levels. pH levels should be between 7.4–7.6 ideally. If it is too high or low, based on the gallon size of your pool they will be able to tell you how much of what product to add, (i.e. muriatic acid to lower or sodium bicarbonate to raise depending on your alkalinity reading).
  3. Check your Alkalinity. Alkalinity levels should be between 80–120ppm.

What Do I Do After I Get My Pool Water Tested?

If your chlorine, pH, and alkalinity are within the recommended range, then there are two other things that could, potentially, be causing your problems.

  • Phosphates: Phosphorus commonly enters the pool through wind drift. Do you have a lawn company that sprays near the pool? Phosphates can drift into the water on which algae feed. If the phosphate level is high, you will need a bottle of phosphate remover such as "PhosFree".
  • Cyanuric Acid: This is one of the most common problems I run into. If your pool uses Chlorine tablets, you will eventually have this problem. If you have a salt generator it won't be an issue. Cyanuric acid or "stabilizer" is in chlorine tablets. The water is being fed a continuous amount of stabilizer.

    It's very common to over-stabilize the water. This doesn't happen overnight, it can be a year before the over-stabilization takes place. This means that even though you are getting a high chlorine reading, the chlorine molecules have been locked by the stabilizer and are no longer effective as a sanitizer causing algae to feed. In order to correct this problem, the water will need to be diluted.

    There is no chemical on the market that reduces stabilizers. It can be added, but not removed (except by diluting the water). This is done by draining a few feet of water from the pool and adding fresh water. High calcium and high stabilizer are pretty much the two things that can't be fixed by adding a different chemical.

2. Shock the Pool

Once your pH is 7.2 or below, you are going to start by shocking the pool with granular chlorine (calcium hypo-chloride). I suggest purchasing a 25-pound container of granular chlorine, rather than the individual one-pound bags they sell at pool stores or large chain stores. You'll save a great deal of money, and you will need chlorine in the future.

Use the chlorine to add small doses to the pool from time to time. Use five pounds of granular shock or 10 gallons (four 2.5-gallon jugs) of liquid chlorine. With the filter pump on, broadcast the chlorine evenly over the water covering the entire pool surface until all five pounds of shock or all ten gallons of liquid chlorine have been used.

Be sure to use a good algaecide as well, which you can add after a few hours of circulation.

pool with algae

pool with algae

3. Pump and Filter

What type of filter do you have? Follow these steps for your type of filter:

  • Diatomaceous Earth (DE) Filter: Before anything, backwash the DE filter. Add fresh DE powder, shock pool as explained above, and run the pump for 24 hours. Make sure there are no obstructions on the pool drain, such as leaves.

    Of course, because the pool is green, you can't see the drain, so just run a brush over the approximate place where the main drain is at the deep end of the pool.
  • Sand Filter: Same as DE, except the backwash time should be a minimum of five minutes.
  • Cartridge Filter: Make sure it's in good condition and rinsed thoroughly. See note in Step 4 below about cartridges.

4. Brushing and Filtration

After 24 hours of chemicals and circulation, you will see an amazing transformation. Your pool should not be "green" anymore. But it will still be cloudy, and it will need a lot of brushing and filtration for the next few days.

There will probably still be a few stubborn isolated green areas that need some brushing. After 24 hours of constant circulation, backwash the filter.

Special note for cartridge filters: they will need to be cleaned more often than other filters during this process: twice a day for at least two days, or until the pool is clear.

Using these steps, you should be able to remove the algae from your pool and prevent it from coming back.

Using these steps, you should be able to remove the algae from your pool and prevent it from coming back.

Useful Pool Cleaning Supplies


In The Swim Chlorine Pool Shock


Flexible Spa and Pool Vacuum Head


Aluminum-Back Algae Brush w/ Stainless Steel Bristles


Digital Pool Water Test Kit


Super Concentrated Clarifier


Pool and Spa Test Strips


Gallon Muriatic Acid


Why Am I Getting Algae in My Pool?

  • Your chlorine levels might be too low.
  • Your pH levels might be off.
  • You might have had a lot of guests in a short space of time. This can add massively to the amount of organic material in the water.
  • A period of high winds can mean a lot of extra dust and plant material finding its way into the pool.
  • In the fall, dead leaves can be a problem.
  • Even pollen can have a big impact at certain times of the year.
  • In short, your pool's chemical balance is off.

What Are Algae?

I will cover only two of the two common types of algae for now. Green algae and yellow (mustard) algae. These can be caused by a number of things, the first being low chlorine or sanitizer.

  • Green algae are free-floating algae. While they may hover slightly around the sides of a pool or the steps, they are not actually attached to anything. Green algae are most common because they are very opportunistic, meaning any faltering in your pool's chemistry or care will usually spark green algae's presence.
  • Yellow or mustard algae is similar to green algae but lacks chlorophyll, the chemical that produces the green pigment in plants. These algae are less common because they take a longer time to develop. However, once mustard algae take hold, it can be difficult to treat, sometimes requiring multiple treatments to fully eliminate.
This is the kind of water clarity every pool owner dreams of—and it's achievable, if you follow the instructions in this article.

This is the kind of water clarity every pool owner dreams of—and it's achievable, if you follow the instructions in this article.

Further Reading

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

Question: I have an above ground vinyl pool. I have put Clear Aid in the pool to clean, but it is still green. What can I do?

Answer: Above ground pools can be more difficult to clear because of smaller filtration systems. For best results, use a floccing agent. A quart of "Drop-out" or "drop-down" will work by settling everything to the bottom of the pool, including all of the "green" and then can be vacuumed to waste (not back to the pool). You'll have a few inches of water loss using this method, but it's usually very effective. To use a floccing agent, elevate the chlorine to 10ppm. Brush the pool. Add a quart of Drop-Down. Run the pump for 2 hours, then shut off the pump for 24 hours. Everything should sink to the bottom. Vacuum to waste then re-balance water chemistry.

© 2010 Rob Hampton


Rob Hampton (author) from Port Richey, Florida on May 30, 2014:

Hey Chad, thanks for the comment. You will raise the alkalinity using sodium bicarbonate (baking soda) don't confuse this with soda ash which is used for pH. If your alkalinity is very low, you can use quite a bit of bicarbonate. It's not a harsh chemical and takes quite a bit to raise alkalinity. Start by diluting 2 pounds in a bucket of water and throwing it in the pool. Let circulate for a few hours then retest. Add as needed. Muriatic acid will lower alkalinity, but is a double edged sword as it will also drop the pH. High alkalinity will usually come down on its own if the pH is balanced.

Chad on May 30, 2014:

How would one go about raising or lowering alkalinity? I see the test for it but no cure.


Premier Robotic on February 05, 2013:

Invest in a good robotic pool cleaner! This will not only prevent an algae attack from coming on, but also if you have a really GOOD cleaner, it can help treat an attack already underway.

Personally, I use the Dolphin Premier by the folks at Maytronics. It has tons of features I haven't seen on others (smart nav technology, a 360 degree cable swivel, multimedia options and much more...) and it actually does a good job on the waterline and algae-- something I haven't seen in many others out there.

So if you're interested, check them out at!

Thanks for the post.