Jays Precision Pools- Why Is My Pool Green

Why Is My Pool Green?

We are guessing that this is not a design choice on the part of the pool owner.  And yes, we get this question all the time in the summer when everyone is using their pool.  It’s amazing how a pool can go green in what seems like overnight but often, this is something that grows for a while.  This something is usually algae.  The good news is we know what it is and what the remedy is but, let’s look into this issue a little further so that we can try to prevent it BEFORE it becomes an issue.

Any quick google search will yield endless results of how to get rid of algae so knowing what it is that causes a pool to turn green is not the problem.  The problem is why the pool turned green.  There are many factors as to why this can happen.  Let’s star this discussion by looking at where this algae comes from, shall we?

Algae?  In My Pool?!

For starters, the lack of chlorine in the pool is usually where all the problems start.  It is known that chlorine is the primary chemical that is used in pool water chemistry to keep it clean and clear of living organisms such as germs and algae.  Algae will start to grow when the disinfecting power of the chlorine is diminished either by time or sun evaporation.  The lower the chlorine in the pool, the more the algae growth explodes.  But, algae can grow from other issues besides weak or lacking chlorine.  Sometimes it has to do with your pool pump and filter being too small.  Other times it is that your pump is not running long enough to filter out all of the algae growth and other contaminates.  So, how do you start checking the pump and filter if your chem levels are solid?

Pump and Filter

After you are sure that this problem is not caused by lack of chlorine, the next thing to check would naturally be your filter and pump size.  If you are a new pool owner, this may come as a surprise to you but it is entirely possible to have a pool with poorly sized pump and filters installed.  Generally speaking, if algal growth is associated with one piece of equipment it will be the filter.

The first step in checking your filter is to verify how many gallons of water are in the pool.  This can be tricky as you will need to do some math.  Each pool shape has a different formula to calculate this.  When you check the size of your pool filter, you want to be sure that it is large enough to handle the size of the pool.  You also need to be certain that you are running your pump long enough to filter your whole pool.  It is possible to have a filter that is too small but a pool that is still blue but we can guarantee that you or your pool professional is clearing that filter frequently.

How will you know if you are running your pump long enough?  Well, once you know how many gallons of water your pool holds and you verify the filter size, you can figure this out.  This is the turnover rate.  A turnover rate is how many gallons of water filter through your pump within a certain amount of time.  The standard rule is that you want the entire pool to be filtered through the system at least once every 8-10 hours.  If you take a look at your filter manual, it will tell you what this turnover rate should be.

Here is an example:  Your pool has 25,000 gallons and your filter can has a turnover rate of 29,000 gallons in an 8 hour period.  With this example, if you are running your pump for at least 8 hours a day you should be fine as the filter can handle this pool size just fine.  However with a larger filter, you would not have to run the filter as long because it can handle more gallons per hours.  We know that the goal is to run the pump as few hours as possible in order to save money but you must be certain that you are running your pump long enough to filter out all of the contaminants and bacteria to avoid a build up.

Test and Balance Your Pool Chemistry

Once you have determined that your pool filter and pump are the correct size and that you are running the pump for enough time to filter out all contaminants and that your filter is actually clean, we now need to look at your pool chemistry.  When was the last time you checked the chemistry and do you keep a record of it?


This is where you need to pay very close attention.  The lack of chlorine levels is in fact, the leading cause of green pools!  Chlorine is the sanitizer that keeps bacteria and other contaminants at bay.  When the contaminants build up…BOOM algae blooms!  If your chlorine levels remain too low for too long, algae is the least of your problems!

Cyanuric Acid

Chlorine gets depleted by interacting with germs and bacteria but also consumed by sunlight.  Studies have proven that 90% of unprotected chlorine is lost within 2 hours of direct sunlight exposure.  The Cyanuric Acid acts as a reservoir for the chlorine.  It is important to keep the level of the CYA in ratio to the chlorine so that the chlorine can do it’s job by killing the bacteria and germs that cause all of our swimming pool woes.  General recommendation is that you keep your chlorine level about 7.5% of your CYA level.


Understanding why your pool is growing algae is half the battle.  Once we know what the root cause is, we can help you fix the problem in no time!  However, it is best practice to make sure that you are maintaining your pool and checking all parts and chemistry at least weekly.  This will spot a problem before it becomes massive and save you time and money!  For more information or to speak to us about chemicals or pool parts, contact us today!

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