Algae is dreaded and feared among pool owners everywhere. Fear not as this guide will teach you how to clean your green pool and keep it crystal clear for years to come. 

Regardless of if it is a small algae outbreak, a full-blown swamp, green algae, yellow algae, or even stubborn black algae, the answer to cleaning your pool is simple: chlorine and elbow grease. 

I will teach you the basics about algae and walk you through cleaning your pool every step of the way. 

What is Algae?

Algae is simply a generic term for a wide variety of aquatic organisms and can come in several forms in your pool. 

The most common is green algae but also can be yellow/mustard algae and even black algae. Technically, black algae is a type of bacteria called cyanobacteria but for all practical purposes, we treat black algae just like any other form of algae found in your pool. 

Is algae harmful?

In general, no, a small patch of algae on a wall of your pool is not directly harmful to your health. 

However, a pool environment growing algae is also capable of growing things that are directly harmful to your health like cryptosporidium, pseudomonas, legionella, and all sorts of other organisms that you do not want to be around.  Think of algae as your canary in the coal mine, as algae can alert you to the dangers in your pool. 

Why did I get algae?

Your pool is constantly bombarded by threats from people swimming, things blown in by the wind, birds flying overhead, etc.  All of these need to be neutralized by your sanitizer and if you do not have enough sanitizer in your pool these threats can take hold and thrive to the point where they are a visible problem. 

Simply put, there was not enough sanitizer in your pool. 

What kills algae?

Chlorine is king when it comes to sanitizing your pool and has been used for hundreds of years to keep water clean. Chlorine is EPA approved and even found in your drinking water. 

Chlorine comes in many forms from liquid, to solid pucks, to powder, to even being produced by your pool equipment in a saltwater pool. Chlorine is available in different strengths so make sure you look at the ingredients list, for liquid chlorine and bleach you need to look at the % of Sodium Hypochlorite to see the strength.

Copper-based algaecide can also be used to kill algae in pools, but I do not recommend using that since it does not completely sanitize your pool and can also cause staining of your pool, hair, and other issues down the road.  For cleaning a green pool, stick with chlorine. 

A common misconception is that your filter kills algae. Your filter plays an important role in the cleanup process, mainly filtering out dead algae particles from your water, but it certainly does not kill algae. Algae is perfectly happy living suspended in your water, on the walls of your pool, and within your filter. 

How to Clean a Green Pool: Step by Step Guide

Prepare

To prepare to clean your green pool, you will need to do the following:

• Clear any debris out of the bottom of your pool. This can be back-breaking work, but will pay off in the end and let your chlorine focus on killing your algae and not try to fight algae and a year’s worth of leaves at the bottom of the pool.

• Secure a good source of chlorine, lots of chlorine. I prefer liquid chlorine as that just adds chlorine to your pool, as opposed to solid forms of chlorine like powders and pucks that contain CYA and Calcium. 

My favorite liquid chlorine is Pool Essentials Chlorinating Liquid, which can be found in the pool section of Walmart and many other hardware stores or even online: 

Even if you have a saltwater pool that makes its own chlorine, you still need to add additional chlorine while you are cleaning your green pool. 

• Have a plan to test your water frequently and at elevated chlorine levels. You can do it yourself or have a pool store do it for you. I recommend a test kit that contains a FAS DPD test that allows you to test chlorine at levels above 5, and will test for CYA like this: 

Step 1: Get your baseline test results

Test your water using your preferred testing method mentioned above. The key things to test for at this point are your cyanuric acid (aka CYA or stabilizer), your pH level, and your free chlorine (FC) level. 

Step 2: Adjust your levels

First of all, review your CYA level from step 1 as you want this number to be between 30 and 80.  If it is too low, simply add CYA. 

If it is too high, unfortunately, you will have to drain some of the water out of your pool, proportionate to your decrease. For example, if you have a CYA of 120, you would need to drain 50% of your pool to lower your CYA to 60.  Note, your pool is designed to have water in it at all times so I never recommend draining more than 50% at any given time. 

Next, adjust your pH level to 7.2. Standard pH testing is not accurate at elevated chlorine levels, so lowering your pH to 7.2 ensures your pH remains below 8.0 while you are cleaning your green pool.

Step 3: Shock your pool

Now you are ready to get to work killing algae.  To do this, you need your CYA number to determine how much chlorine to add. 

Simply multiply your CYA number by 40% and that is your target shock level. For example, if your CYA is 50, you need to bring your free chlorine up to 20.  Do not worry if this chlorine level is shockingly high, as the FC to CYA ratio is a proven ratio been proven over the last 20 years.

Liquid chlorine is the preferred method of shocking since it is instantly available and does not include any unnecessary ingredients. To add liquid chlorine simply pour the desired amount into the deep end with the pool running.  Aim for pouring out a finger-sized stream from your bottle or measuring cup.

Step 4: Keep shocking your pool

Remember when I said to prepare to use lots of chlorine, this step is why.  You need to continue to keep your chlorine at the elevated shock level until all of your algae is dead and gone. 

For best results, you should test and add chlorine at least twice a day. This step may continue for several days or even a week or more depending on how much algae you have in your pool. 

Keep in mind that your pool did not turn green overnight, and will not turn back to crystal clear overnight either. 

Step 5: Other recommended things to do, while you wait on step 4. 

Step 4 is the longest in the entire process, but there are other things you can do to help kill algae during this time. 

I recommend brushing your pool at least once a day. Algae forms a protective layer over itself to protect it from chlorine, which is why algae can feel slimy. 

Removing this protective layer from the algae helps the chlorine get to the algae and do its job.  This is especially important with black algae and may even require a wire brush to carefully remove the protective layer.    

Make sure you investigate algae hiding spots like the skimmer weir doors, ladders, stairs, drains, and even lights. It is also good to run waterfalls, spas, and other water features for at least a few minutes each day to make sure they get freshly chlorinated water as well. 

Weir Door, a favorite hiding spot for algae that is often missed:

I also recommend that you run your pool 24/7 until it is clear. While chlorine is king at killing algae, your filter is king at removing the dead algae from the water.  The more times your water passes through your filter during this step the quicker it will begin to clear. 

Speaking of your filter, keep an eye on your pressure gauge and clean or backwash your filter when needed as it will quickly fill up with dead algae.

The general rule of thumb is that when your psi rises 20-25% from the psi when your filter is clean, it is time to clean it again. This may be every couple of days and I have even seen some cases that required daily filter cleaning. 

Lastly, make sure you are removing any leaves and other debris in your pool that you may have missed when preparing. 

Do not worry, everyone misses spots when the water is murky. As your water clears, you are bound to see more debris on the bottom of the pool.  The quicker you get that out of there, the more chlorine there is to fight the algae. 

Step 6: What NOT to do

You have a lot to do in steps 4 and 5 and it is always tempting to use shortcuts such as algaecide, flocculants, clarifiers, etc.  I do not recommend those products as they can gum up your filter and cause other long-term problems down the road. 

If you are bored, re-read steps 4 and 5 as those are what will help speed your progress along. 

Step 7: Knowing when to stop shocking your pool

Typically a green pool will transition from a green swamp to a slightly better looking blue cloudy pool, eventually to something that resembles a pool, and finally to a crystal clear pool. 

How do you define crystal clear? I have heard it described as being able to call heads or tails on a quarter thrown into the deep end. 

My clients typically do not like me throwing coins into their pool so I make this call by looking at the deepest part of the pool, which typically has a drain.  f I can tell if the drain screws are Phillips or flathead, then that is crystal clear. If I cannot, I keep at it until it is that clear. 

Here is a picture of a client’s pool drain taken standing on the side of the pool and looking down at their drain 6 feet below the surface:

You can also measure your chlorine loss overnight to determine if your chlorine is done killing algae. Do a chlorine test after sundown, then another before the sun rises.  If you lost less than 1 FC, then you can safely say that your chlorine is not battling any more algae.  More than 1 FC means it is fighting something and you need to keep at it.  

Do not cut this step short, as doing so can cause algae to quickly come back.  ou worked hard to get here, so make sure you complete this step. 

Once you are satisfied that all algae is gone from your pool, you can stop shocking your pool and let the chlorine levels drift down to your target levels. 

As your chlorine levels are drifting down, know that it is safe for you and your family to swim in water that is lower than your elevated FC level. That means pat yourself on the back and jump in!

How to Keep Your Pool Clean

Test often

Test your pool daily until you get a feel of your pool, then test at least 2 times a week. Temperature, number of swimmers, and even the weather impact your daily chlorine needs so you want to make sure you are keeping up and keeping the algae away. 

Brush

I recommend brushing your pool weekly.  This helps circulate the water, allows dirt to be filtered out of your pool, and disturbs any algae that is trying to form in your pool. 

Properly Maintain chlorine

Remember, chlorine is the best sanitizer for your pool and you need to make sure that your pool has enough chlorine to stay clean and clear.  Your target chlorine level is calculated using your CYA, similar to what you did in step 3.  The only difference is that your chlorine can be much lower now that your pool is crystal clear. 

To calculate your target chlorine levels, multiply your CYA level by 11.5%. So a CYA of 40 would target an FC of 4.6. 

That means you want your pool to stay near that target level most of the day. In the above example, it would be perfectly fine to raise your chlorine to 5 or 6 so that it can drift down to the target level over the day. 

You can also calculate your bare minimum FC levels using your CYA. Simply multiply the CYA level by 7.5% and that is your minimum FC level to never go below. Going below this number will allow algae to survive in your water and we do not want that do we?

FAQ

Can you clean a green pool with bleach?

Answer:  Yes you can. Bleach is liquid chlorine and should be easy to find. Just avoid any extras to your bleach like scents, additives to make it splash less, etc. 

What do I do if the chlorine level is high?

Answer: Chlorine is consumable and used up while it is doing its job of sanitizing your pool as well as being used up by the sun. If your chlorine is truly high, defined as >40% of your CYA level you can usually let nature take its course to quickly bring down your chlorine levels by stopping adding chlorine and remove any pool covers that may be blocking the sunlight. 

In the very rare case that you messed up and added way too much chlorine and need to make a quick adjustment, I recommend chlorine neutralizers that contain sodium thiosulfate. 

Do I need to drain my green pool beforehand?

Sometimes. I do not recommend draining a pool for algae control, but many times the pool requires lowering of CYA or calcium which is accomplished by draining and refilling. 

I do not recommend ever draining more than 50% of your pool at any given time as your pool is designed to have water in it and draining all of the water out can cause issues and even pop in-ground pools out of the ground. 

Can I clean my green pool without a pump?

Your pump is part of the entire cleaning process and is needed for your pool to stay clean.  If you find yourself without a pump for a short period, you can still kill and prevent algae by adding liquid chlorine to your pool. 

Just make sure and brush your pool afterward to help distribute the chlorine evenly. 

Can I clean my green pool without chemicals?

No, your pool needs a sanitizer to kill the algae and other organisms. 

I recommend avoiding many pool chemicals on the market, but you still need the basics of chlorine and chemicals for the occasional pH adjustments.  Even systems like UV and Ozone sanitizers require chlorine for residual sanitization. 

Wrapping It Up

I hope this guide gives you the confidence and tools to clean your green pool.  Remember, chlorine is key to killing and preventing algae and the amount of chlorine your pool needs is driven by your CYA level: 

To prevent algae you need FC that is at least 11.5% of your CYA.

To completely kill algae bump up your chlorine to 40% of your CYA and keep it there until all of your algae is dead.