Green Pond Water

You are currently viewing Green Pond Water

Green Pond Water Is Frustrating!

As the weather gets warmer, your pond seems to be getting greener. No matter what you seem to do, it happens every year. Algae is the most unique organism on planet earth. It grows in all 7 continents, whether you have freshwater, brackish water, or salt water even. It’ll even grow in cold water or hot water. There are so many types of algae, hundreds of thousands of species that we can’t even count them all. Now, no one wants algae in their pond, but having algae in your pond is actually a good sign, but too much of something can also be bad, so lets dig in a little deeper on why you may have green pond water. There are two types of algae that you will usually find in your pond. The first one we will talk about is the planktonic algae, which is the reason your pond is green because the algae is suspended within the water molecule. The second type of algae is a filamentous type, usually known as string algae, usually growing around rocks in a branch-like form.

The reason algae is growing in your pond is because the beneficial bacteria in the biological filtration is not absorbing all of your excess nitrates and phosphorous therefore the algae has a chance to grow and thrive. Algae will particularly thrive in a hot sunny day, because algae is a photosynthetic plant. Now you may be tempted to use pool chemicals to solve your problems, the issue with that is that you will probably kill your fish and there is no way to just target the floating algae (planktonic algae). We want to conserve the epilithic algae, the algae which grow on rock and gravel, as that is a source of our biological filtration within the ecosystem and the reason why we’ll have clear water. Now if you have a brand new ecosystem pond, you will 100% go through cycles of where your water will be green and then clear up until your ecosystem establishes, this is normal behavior.

Phosphorus Levels

If you’re pond has a leak, or it’s been especially hot and you’ve been filling up your pond with city water then the first thing you should do, is grab a water test kit that has a test for phosphorous. Test the city water from the hose for phosphorous because chances are that you’re adding additional phosphorous to your pond causing the algae bloom. Phosphorous is commonly used in fertilizer as it has a tendency to cause plants to grow extremely well. If you aren’t filling up your pond with water constantly, but test your pond and find phosphorous levels to be high then you may want to investigate the type of fish food you have. It’s very common for koi food to high a high phosphorous level. At this point you may want to invest in a food with low phosphorous levels and stop feeding your fish so frequently until you get the pond under control. Another area to look into would be any runoff of water into the pond. Do you have a slope behind the pond that dumps water into the pond when it rains? You may be adding additional fertilizer to the pond without even knowing it causing your algae bloom. Some solutions to look into would be to add a phosphate binder to the pond and make sure you don’t keep adding any additional water until your problem is resolved. 


So you use the water test kit and find that you have a high level of nitrogen, whether it be nitrites or nitrates. High level of nitrates create a situation where algae growth will go unchecked. Not only is excess nitrogen in your pond bad for the algae reason but it is also bad for the health of your precious fish and can be fatal in high levels. You will need to find the cause of your high nitrogen levels. Whether it be from having runoff into the pond, having grass clippings fall in the pond, excess fertilizer in your lily pad pot, or having a pond that is over stocked with fish. A good rule of thumb for koi fish is 1 inch of fish per 10 gallons of water. If you find yourself way over this rule of thumb then it may be time to increase the size of the pond, increase the filtration or get rid of some fish. A fun fact to help you understand the demand of Koi fish is that a 13″ Koi will approximately weigh 1 lb, but a 26″ Koi is around 13 lbs. Double the size but 10 times the mass, and that is why you’re overstocked pond may be fine when your Koi fish are little but a disaster when they are older. Another good idea would be to add fast-growing plants in the pond, such as hyacinth, water lettuce, elephant ears to just name a few, to absorb all the excess nutrients as well as adding a healthy dose of beneficial bacteria to out compete the algae. Try to find out why you have excess nitrates before you go adding random water treatments in your pond.

Shading The Pond

Now this may be hard to do depending on the size of your pond, but if you can shade your pond then you reduce the amount of sunlight that the algae can take in and decrease the reproduction of algae since algae is a photosynthetic plant. Whether the solution is using an umbrella, or using floating aquatic plants to cover the pond with, it will help with keeping algae numbers down. Another choice would be to use a pond dye, either black or blue, to help shade the pond. An additional plus will be that the temperature of the water will decrease and therefore the dissolved oxygen will increase as water molecules can hold more oxygen when the temperature is lower. 


Investing in an aerator on top of having a waterfall is never a bad idea especially when you have algae issues. The reason algae grows in high numbers is due to excess nutrients whether it is from fish waste, fertilizers, fallen leaves etc. Since algae is a photosynthetic plant, it produces oxygen and takes in carbon dioxide during the day but stops during the night when photosynethis is not happening. However at night, dissolved oxygen may decrease to low levels as a result of decomposer bacteria feeding on dead decaying algae. The presence of such organisms increase the BOD (Biochemical oxygen demand), therefore the higher the BOD the lower the oxygen available to your fish. Your fish need a dissolved oxygen level above 6 parts per million to thrive. 

Cleaning Your Pond

Now none of this will work if you have a layer of sludge or muck at the bottom of your pond that the algae is feeding on. You’ll need to do a thorough pond cleaning yourself or hire a professional to do it for you. Here at Tranquilityscapes we service and clean all ponds on Long Island. If you want to consult with us to clean your pond or figure out your algae issues click here.

Treatments for Green Pond Water

Now when treating for green pond water, we want to work with mother nature not against her. Therefore, we don’t recommended using harsh chemicals that will destroy your ecosystem balance or possibly kill your fish.

  • Flocculant & Fine Filter Pads
    • Algae is such a fine particle that filters can not catch it, therefore by using a Flocculant you’re binding multiple algae particles together until they get big enough to be caught by a filter pad. That is why we recommend using a very fine filter pad, and place it underneath the waterfall and you’ll be surprised how fast it turns green/brown.
  • Beneficial Bacteria
    • This helps outcompete the algae and eats up all the excess nutrients in your pond.
  • Maintain for Ponds
    • This formula contains your beneficial bacteria, phosphate binder, flocculent, and a pond detoxifier in one fixing all the issues you’ll have.
  • Automatic Dosing System
    • An electronically-operated dispenser that accurately and consistently applies your formulated water treatment. Perfect for hands off maintenance and problem solving and will keep your water in pristine quality year round.

String Algae

Unlike planktonic algae, string algae is expected to be in every pond and is a normal and healthy sign of a functioning ecosystem. Luckily it is easily removed and not an issue if you find it unsightly. 

  • IonGen System G2
    • Controls algae by using copper molecules through its ion-producing probe. To be turned on only when you have string algae problems and then turned off when you don’t. You do NOT want to leave this on all the time due to copper in high levels can be bad for your fish.

We are here for you if you have any questions or concerns, or even if you want a professional to take care of the issue for you please contact us