How to Get Rid of Green Water Pond Algae


Green pond water won't hurt your fish. But that doesn't mean you have to live with it.

At Splash, all of our ponds are crystal-clear to the bottom all year round, without the use of harsh algaecides or expensive UV clarifiers. We use the same techniques to keep all of the ponds we design and build looking beautiful.

So how do we do it? Read on ...


At a Glance: How to Get Rid of Green Pond Water

  • The best way to get rid of green-water algae is to take away its food source: nitrites.  (Nitrites come from excess debris and ammonia.)
  • Beneficial bacteria consume nitrites before single cell green-water algae has a chance to thrive.
  • A healthy population of fresh beneficial bacteria will keep your water crystal-clear without the use of algaecides or UV clarifiers.

Step 1: Skimming the Pond Physically Removes Excess Nutrients

Green Pond Water Algae

First, let's talk a little about the reason your water turns green in the first place: green water algae.

What is green water algae? Green water algae is a single-cell organism that lives in the water and turns it pea-soup green. Your hand might disappear if you reach into the water, but you can't physically pick up the single cell algae.  It's just too small. (This is different from string algae, which is the gunk you can physically hold).

Where does this green water algae come from?  Single cell algae is one of the most common organisms in the world.  It will thrive anywhere there are excess nutrients (nitrites).  In your pond these nitrites derive from the process of breaking down ammonia.

So where does that ammonia come from?  Leaves, twigs, sticks, pollen, fish waste and any other organic debris that gets into the pond.  In a closed ecosystem pond all of these things enter the pond from the surface.  Your pond acts like a magnet, seemingly pulling these debris from all around.  As you can imagine, the best way to reduce ammonia is skimming this debris out of the pond before it can begin breaking down and releasing harmful ammonia into your ecosystem. A properly designed and functioning ecosystem pond will use skimming filtration 24 hours a day to physically remove up to 90% of this debris before it really even starts to interact with your ecosystem.  Check out this page for more information on pond skimming filtration systems.

Step 2: Grow Nitrite-Eating Bacteria

Just like we might love an ice cream sundae or a big juicy bacon cheeseburger,  green water algae thrives on the nitrites that build up in your pond.  At moderate levels, these nitrites aren't necessarily bad for your pond - in fact, they're a natural byproduct of beneficial bacteria  breaking down the ammonia that would make your fish very unhappy.  We've seen many examples of ponds with green water where the fish seem content.  It's just that most people seem to want the water to be clear so they can enjoy watching their beautiful fish as they dance through the water. The easiest and most natural way to get rid of green water algae is to remove its food source - debris, ammonia and nitrites. 

The most common bacteria that oxidizes ammonia is Nitrosomonas.  Getting rid of ammonia is critical in keeping your fish happy.  If you want crystal-clear water, however, you'll need to grow other types of beneficial bacteria that will convert the excess nitrites before the green water algae has a chance to thrive.  The most common group of bacteria that convert nitrite to nitrate are in the genus Nitrobacter or more recent research points to Nitrospira.

So how do you grow bacteria?  The second step (after making sure your pond skimmer is doing it's job) is installing a properly sized biofilter.  Biofilters are specifically designed with lots of nooks and crannies.  They typically contain media like filter mats, lava rock, BioBalls, bio ribbon and other things that have lots of surface area where beneficial bacteria can colonize, reproduce and grow.  This is where we see the ammonia-loving bacteria consuming the ammonia and producing nitrites.  If your filter is large enough, and you do not disturb the bacteria colonies, your filter will also grow the second group of beneficial bacteria that will consume the nitrites and produce nitrates. We recommend adding a small dose of fresh beneficial bacteria weekly or more frequently.  Bacteria need time to grow, which is why newly built ponds or newly cleaned biofilters are especially susceptible to green water algae.

Ecosystem Pond Algae Control

The second key to growing beneficial bacteria in an Ecosystem Pond is to constantly add a little dose of fresh bacteria on a regular basis. You can do this manually once per week using a pump bottle you buy from Splash, or you can take care of it automatically with autodosers.

Once you have all that good bacteria in your pond, make sure you keep it there. Only clean your biofilter once a year - usually in the early spring, before the pond fully emerges from winter dormancy - so you don't wipe out established bacteria colonies. If you must clean it more often, gently rinse it with water from your pond to make the process as gentle as possible.

Don't confuse your biofilter for your skimmer, which sucks in debris like fish waste and dead leaves so it doesn't decay in your pond. You can clean your skimmer as much as you'd like during the season - most people do it about once per week.

Step 3: Enjoy Your Clear Water

If you stop by the Splash Design Center, you'll see that all our ponds are crystal-clear to the bottom. It's not because of anything magic that we're doing other than putting beneficial bacteria in the ponds.

So let's recap: Green-water algae is caused by excess nitrites. We can either feed these nitrites to the green water and enjoy a green water pond, or we can feed these nitrites to a beneficial bacteria that's growing in your biofilter. If you take good care of your beneficial bacteria - by having a large-enough biofilter, seldomly cleaning that biofilter and building the bacteria colonies with weekly water treatments - your green water problems will go away.

Still have questions about green water in your pond? Don't hesitate to reach out to us at Splash Supply Co.

A Note About Algae Treatments

You might notice we didn't mention algaecides or UV clarifiers as part of our solution for green water algae. The simple truth is this: You shouldn't need them.

Algaecide will kill the algae, but can also kill your fish if you don't carefully follow dosing instructions. It also does nothing to treat the root of your algae problems - excess nitrites - dragging you into a cycle of constantly buying more chemicals to keep your water clear.

UV clarifiers and sterilizers, on the other hand, don't pose any risk to your pond life. They generally work by moving your water past a UV light that damages algae cells and stops them from reproducing. UV lights will consequently keep your water clear, but, like algaecides, do nothing to treat the cause of green water algae.

We sell both algaecide and UV clarifiers at Splash for pond owners who want to try them. Be aware, however, that your pond still needs beneficial bacteria to stay clear in the long run.

Learn More About Pond Algae

3 Things to Know About Pond Algae

Pond Ecosystems: How Skimmers, Biological Filters Keep Water Clear

10 Ways to Keep Pond Water Clear

Thinking About Washing Your Pond Rocks? Don't

What Causes Pond Algae?

How Can I Keep Algae Out of My Pond?

Do UV Lights Destroy Algae?

How to Get Rid of String Algae in Your Pond

The Ultimate Guide to Clear Pond Water

FREE SHIPPING on winter pond gear at our Online Store

Shop Now 

FREE SHIPPING on winter pond gear at our Online Store

Shop Now