How to Clean a Green Pool

Step-by-step instructions to SLAM your pool!

Sometimes it’s inevitable that you get an algae bloom. This article details how to clean a green pool.

Note: There are many different ways to clean a green pool There are different chemicals you can use, products you can buy, etc. In this particular case, we chose to use liquid chlorine because of our initial chemical levels.

Clearing an algae bloom is sometimes referred to as shocking or slamming your pool. SLAM stands for Shock Level and Maintain, because we need to raise the chlorine levels high enough to kill algae, and keep it there until all the algae is dead and the pool is clear.

How to clean a green pool

You will want to keep your pool at shock level until ALL of the following requirements are met:

  • Combined Chlorine reading of 0.5ppm or less
  • And your pool water is clear
  • Pass overnight free chlorine loss test (OCLT) – more on this later in the video

Equipment needed to clean a green swimming pool:

*You do not need a manual vacuum attachment, but it will make the process much easier!

You will need to run your pump 24/7, and do the following each day:

  • Vacuum debris from pool floor (to waste)
  • Brush pool walls
  • Test FC level (at least 2 times per day, but 3-4 times is ideal)
  • Backwash/clean your filter

Recommended Chemical Levels

With Salt Chlorine Generator
Without Salt Chlorine Generator
Free Chlorine
3 – 7
3 – 7
7.4 – 7.7
7.4 – 7.7
Total Alkalinity
70 – 90+
60 – 80
Calcium Hardness
250 – 350*
250 – 350*
Cyanuric Acid
30 – 50
70 – 80

*For pools with vinyl liners, you do not need any Calcium Hardness (CH). The recommended range is 0 – 350.

Step 1: Test Your Chemical Levels

Before you go out and buy chemicals, it is best to test your water chemistry  so that you know what chemicals you need. The FC (free chlorine) needed to raise your pool to shock level will depend on your CYA level.

Step 2: Purchase Needed  Chemicals (Based on current levels & Pool Calculator)

You will need to bring your pool up to shock level to clean a green pool. Determine how much you need to raise FC using this chart. Then use the Pool Chemical Calculator to see how much chlorine you need to add to your pool. You will also need to determine the size (gallons) of your pool by using a Pool Volume Calculator.

Different ways to raise your FC:

If your CYA is already high, you should not use granular chlorine. If your CH is high, you should not use Cal-Hypo.

Step 3: Backwash / Clean Filter

Each morning, you will need to backwash your (sand / DE) filter, or clean your (cartridge) filter. Do this before you raise your FC to shock level so you do not waste any chlorine.

Step 4: Vacuum Any Debris on Pool Floor (to waste)

As you start killing algae, it is common that it settles on the pool floor. Use a manual vacuum attachment and vacuum as much as you can to waste each day. If you have time, doing this several times per day can help speed up the process. You want to be sure to vacuum to waste to preserve your filter as much as possible. To vacuum to waste, simply set your filter to ‘backwash’ while using the vacuum.

Step 5: Add Chlorine to Reach Shock Level (Use CYA-FC Chart & Pool Calculator)

After testing your FC (with a FAS-TPD test kit), use the Pool Calculator to and CYA-FC chart to determine how much chlorine is needed to reach shock level.

Make sure you enter the correct size (gallons), current FC, target FC, and % strength of chlorine you are using.

Step 6: Brush Pool Walls

It is important to brush your pool walls thoroughly every day. You want to agitate the surface to loosen any algae that might be stuck to the pool walls. If you skip this step, it will take much longer to clear your algae bloom. By doing this right after adding chemicals, you are also helping mix the chlorine into your pool water.

It is completely normal for your pool water to turn a little murky / cloudy after brushing. 

Step 7: Re-Test FC and Repeat Steps 3-6 Every Day Until You are Finished SLAMing the pool (clear water, CC of 0.5ppm or less, and pass OCLT)

If you purchased enough chemicals when you started, you should not have to make any trips to the store. Hopefully you have enough chlorine on hand to raise your pool back to shock level.

At a bare minimum, you should test the FC level two times per day, though 3-4 times is ideal. Each time you test, use the pool calculator to determine how much chlorine to add (to return to shock level).

Step 8: Preform Overnight Free-Chlorine Loss Test (OCLT) When the Pool Looks Clear

The OCLT is designed to find out if you still have algae in your pool. Since chlorine is used up by either breaking down organic matter, or burn-off from the sun, you want to test the chlorine when there is no sun present.

Test your Free Chlorine (FC) after the sun goes down, and then again the next day before the sun comes up. If the two tests yield the same results, this means you no longer have algae for the chlorine to kill.

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When Kevin’s not enjoying the water himself he’s engaging himself in the wide world of pool and spa knowledge which he passes on to our customers through producing content for our blogs and website. Outside of the pool world you can find him enjoying life with friends, family and his off the wall dog Brixton.