Why Your Pool Is Green and What to Do About It

Imagine waking up in the morning and going out back to enjoy your cup of coffee.  Or maybe you enjoy an early morning dip.  Perhaps it’s a holiday weekend and you are having THE pool party. But then..wait—no, it can’t be…  NOT A GREEN POOL!!!   Well, algae has probably happened to all of us at some time or another, but don’t despair.  We are going to talk about reasons why your pool may have turned green and what exactly you can do about it.

There are several reasons a pool gets an algae bloom.  It can be one, or a combination of reasons.  Let’s dive into some of the more common ones.  (Yes, that pun was intended)

Low/No Sanitizer 

Sanitizer is what we add to our pool water to keep it safe and algae free.  The most common form of sanitizer for an outdoor pool is chlorine, so we will be talking about that.  (But keep in mind these factors can apply  for ALL methods of  sanitizers such as bromine, Baquicil, Ozone and UV).  You may be thinking…wait!  But I have a salt pool!  You have forgotten about me!  Nope.  In actuality, a salt pool is still a chlorine pool.  You just create your own chlorine when the salt is converted by electrolysis as it passes through your salt generator.

When our chlorine gets low or non-existent, we have a “naked” pool, which is a breeding ground for all sorts of nasty things, including, you guessed it, algae.  When our chlorine gets too low it allows algae spores to multiply exceedingly fast.  Did you know that by the time you can actually SEE visible algae growth, it has been going on for days already?  Brushing the pool weekly is an important step in algae prevention because it doesn’t allow the algae to take a foothold.

We can end up with low or no chlorine by not adding it or not adding enough for our pool size.  It can burn off in the sun in a matter of hours in an outdoor pool if we don’t have enough Cyanuric Acid (CYA) present.  (Cyanuric Acid is also referred to as “conditioner” or “stabilizer”.) CYA acts like a sunscreen for your chlorine.   Levels should be maintained at 30-50 ppm for a traditional chlorine outdoor  pool and 50-60 ppm for a salt pool.  Too much CYA can cause your chlorine to be less effective, so we want to keep an eye on it when we are testing our water.

Another way we can lose chlorine is by it simply doing its job.  Heavy bather load and organic and inorganic matter can use up the chlorine in the pool.  “Splash out” is another way to lose chlorine.  Having a leak in your pool or equipment can cause water loss, which can also lower chlorine (and other chemical levels).  

If you have a salt system, you may be low on salt or your cell needs to be cleaned.  If the water level gets too cold (around 58 degrees or less) many salt systems will stop producing and you will have to manually chlorinate until the water warms back up.

You may test your water and say, “Hey, my chlorine level is fine!”.  Well, either at some point it wasn’t, or you have a test kit that is testing for Total Chlorine, which includes chloramines (the ineffective form of chlorine).  You see, when the chlorine does its job and bonds to things such as ammonia or nitrates, we get what is known as “combined chlorine” or “chloramines' '.  Chlorine in this state is useless.  It can also be irritating to the swimmers.  To get rid of chloramines you need to oxidize the pool.  This can be done with either a chlorine or a non-chlorine shock.  This frees up the chlorine to do its job.  We want a “Free” chlorine level of at least 1 ppm (part per million) at all times to have sanitary water.

Poor Circulation

Another culprit of algae is improper circulation.  This can be caused by not running your pump long enough or ,if it is a variable speed pump, too low of a setting (Revolutions Per Minute AKA RPMs).  We need to get enough water through the filter to accomplish what is known as a “turnover” at least once per day.  More during the Summer months, if possible.

You may have a dirty filter.  When the PSI (Pounds Per Square Inch) on your filter gauge reads 8-10 higher than your clean starting pressure, it is time to clean your filter.  If you have a Diatomaceous Earth (DE) filter, sometimes you can backwash to buy yourself some time before a full filter clean.  Make sure to add 75% of the DE required for a full filter clean after backwashing.  Having too much, or not enough, DE can also interfere with filtration.  Your filter may have ripped, torn or old grids or cartridges interfering with proper filtration.  You may have a leak causing air to get into your lines.  Remember, water dripping out when the system is on equals air getting in when the system is off.

A clog in the impeller of your pump or in your plumbing lines can cause an issue as well.  Full pump and skimmer baskets are also sometimes guilty of interrupting your flow.

Unbalanced PH/Alkalinity

I can not stress enough testing your pool water at least once a week.  Especially before adding any chemicals.  You can do this with a test kit, test strips or at your local pool store.  While these test for many levels of different things, we need to keep a closer eye on PH and Alkalinity.  When these categories are either too high or too low, our sanitizer becomes less effective.  We could have a splendid chlorine level, but if it is only functioning an 20% due to an imbalance, what good does that do us?  Remember to check BOTH PH and Alkalinity before adjusting.

Old Water

Swimming pool water should be drained every 5 years, or, when certain chemical levels get too high.  Mainly the total dissolved solids.  When the TDS is too high, it is like too many people trying to get on the 405 freeway when we add chlorine and the chlorine will exhaust itself trying to find room to dissolve.  Always check with a professional and the water company before draining your pool.

How to Get Rid of it

Good news.  You can be rid of this green (or yellow) monster with a few simple steps.

  1. Disconnect the automatic pool cleaner (if you have one), but leave it in the pool.
  2. Run the pump 24/7 (the pump needs to stay on the entire process)
  3. Brush the entire pool
  4. Add x pounds of Calcium Hypochlorite. This depends on the size of the pool.  Typically 2 pounds per 10,000 gallons of water. (For un-painted, plaster pools only)
  • Or x gallons of liquid chlorine. (For all pool types.)
  1. Add x ounces of algaecide poured directly into the pool.  (Dosing depends on the size of the pool and the manufacturer’s instructions found on the product label)
  2. Run the pump/filter for 24 consecutive hours

The Next Day

  1. Clean the filter.  Recharge with DE if that is your filter type.  If the filter was just cleaned, backwash the filter and add DE.
  2. Add x pounds of Calcium Hypochlorite again (unpainted plaster pools only)
  • OR x amount of Liquid chlorine (all pool types) Run the pump at least 2 hours.
  1. Perform a phosphate treatment as needed
  2. Use a clarifier or enzyme as needed to remove dead algae too small for the filter to filter out.

Viola!  Your pool has been restored to blue again!  Hallelujah! See you poolside!