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How Do You Control Algae in a Pond? (Causes and Fixes)

This coming spring, I am putting a Koi pond in my backyard and I am super excited about it! I have a ton of experience with aquariums but have never tackled a pond. I have been researching the answer to every question I could think of on how to set up the pond and what things to look for once it’s ready. It’s not much different than having a large aquarium except that it’s outside. There are certain environmental factors that affect outdoor ponds, including the growth of algae.

How do you control algae in a pond? Algae in a pond can be controlled by using a UV sterilizer, installing a pond filter, cleaning it frequently, providing cover, limiting the amount of food, doing regular water changes and performing daily maintenance. 

Algae is something you just can’t get away from. It doesn’t mean you have to live with it. You can do things to help control it or you can do nothing and let it take over your pond. I am going to review how to control algae in a pond in this article, hoping to help you if you have an existing pond or are thinking of installing one soon.

So, let’s get started.


Why Does My Pond Have So Much Algae? (The Causes)

Before we get into how to control the algae in your pond, let’s discuss most of the more common causes of pond algae appearing. This, in turn, will also help identify how you can control it in the future.

  • If your pond is in full sun, this can cause algae blooms.
  • Poor maintenance will promote algae growth. Make sure you are cleaning the pond on a regular basis.
  • Having a mechanical filter will help clear out some of the excess fish poop and work to create surface agitation so the water gets more oxygen. This supports fish health as well as any live pond plants.
  • Overstocking the pond with too many Koi will eventually cause algae problems. Unless you can work on the pond daily, this is always a bad idea. As nice as an overstocked pond looks, it creates too much nitrate which is dangerous for the fish.


How Do You Control Algae in a Koi Pond?

Piggy backing off the points brought up in the previous section, we can now discuss how you can control the algae growth in your pond.

  • If you have an aquarium that sits in sunlight, you know the struggle with keeping the algae level down. Sunlight will help algae grow, that’s the simple truth. So how do you stop sunlight from shining onto your Koi pond?
    • You can add surface living plants, such as pond lilies and other plants to help stop the sun from shining into the water. You could also try building a gazebo or other structure above the pond to limit how much sun hits the water. Most people want their ponds out in the open, so having plants that cover the water surface is the way to go.


  • Just like with an aquarium, the bottom of a pond needs to be cleaned out somehow. Having a vacuum or large net to scoop out excess poop and food is essential to preventing the growth of algae. This will help to lower the nitrates and in turn, reduce the chances of algae will taking hold.


  • Having filtration on the pond not only gives beneficial bacteria a place to live but is another opportunity to take some of the debris floating around in the water out of the pond. This makes your cleaning routine a bit easier as it helps clean out some of the poop and foods not eaten in the tank. Filtration also allows another opportunity to create flow in the pond which will hopefully cause some surface agitation to allow more oxygen to enter the water.


  • Installing a UV sterilizer can help to eliminate water borne algae. This is the stuff that causes your water to turn green. In fact, if you are going to own a pond, I feel this is a piece of pond equipment that is a must to own.


  • Sometimes it’s just as simple as not overstocking your Koi pond. I know how easy it can be to buy as many beautiful fish as possible. I have done the same thing with my aquariums at one time or another.
    • The best suggestion I can give you is to just stock the pond with a few fish and wait many months before considering adding more. See how the pond is running and if it has any issues with algae or other things. If after 3 to 4 months there is no algae problem, go ahead and add another Koi to the pond. I would never add multiple fish at this point as you just don’t know the ‘tipping point’ until it happens!


Now you have some excellent ideas on how to help you control an algae problem in your pond. Chances are it will happen to you just like it does for everyone else. Doing some preventative maintenance as suggested in this article can really help keep the algae in check.

koi in a pond

The More Common Types of Pond Algae

So, after discussing why you might have algae and how to control it, let’s now touch briefly on the more common types of algae you might encounter. There are more than a few different types of algae but only a few most homeowners with ponds in their backyards really should be concerned with, especially if you live in North America.

Green water algae is very common and one type that most pond keepers will come across. When the pond water has an excess of microorganisms and bacteria, there is a good chance green water will start to appear.

You might think blue-green water algae appears for the same reasons as green water algae but not entirely. If you see your pond water turning a blue-green color, there’s more wrong than just an excess of bacteria. Your pond is not healthy if you see this coloring in the water.

String algae is a type of algae most of us have run into, whether you have a pond or not. String algae (Filamentous algae) is in most lakes that we swim at.  If your pond water doesn’t move enough and it’s sitting in direct sunlight for long periods of time, chances are you are going to see this type of algae.


Is Algae Bad in a Pond?

If you don’t know anything about algae and what purpose it serves, you might just think its a nuisance and we should get rid of it. That’s partially true and partially untrue. Yes, it can become a nuisance when it turns your water green and you have string algae growing from the rocks, but it does have a purpose.

The algae started growing because of an overabundance of organic compounds among other things. It is absorbing those compounds in the pond as well as helping to rid the water of nitrates. This provides much healthier water, believe it or not.

When you are in a big hurry to rid your pond of that pesky green algae, just remember it’s not really all that bad and it’s okay if some stays in the pond.


Do Koi Eat Algae?

If you plan on stocking Koi in your pond, you will be happy to know that Koi will sometimes pick at green algae. There are some nutritional benefits however, I wouldn’t consider algae a food source for your fish. Make sure you are feeding your Koi the nutritional foods they need to have a long healthy and happy life.

Up next, I thought I would answer a few questions about UV sterilizers I found online. I just wrote an article about sterilizers for aquariums which I linked to above and will do some at the bottom of this article again. Everything you read in that article will help you with your sterilizer and a pond. The only difference is you will want a much larger unit for a pond, as a pond will contain many more gallons than a typical aquarium. 


Do I Need a UV Light in a Pond?

I am going to go out on a limb here and say yes, you need a UV light/sterilizer in your pond setup. I would consider a UV sterilizer for a pond as important as a filtration system. Because algae will more easily start appearing in an outdoor pond versus an aquarium inside, it makes more sense to have equipment better to deal with algae. Of course, you can get away without using one, but you will save yourself many hours of time from cleaning the pond if you have one.


Where Do You Put a UV Light in a Pond?

Again, please go and read my article on UV sterilizers for a more, in depth explanation (link below).

UV sterilizers are installed in the filtration system typically on the outline of a canister or pond filter.


How Long Does It Take a UV Sterilizer to Clear Green Pond Water?

Of course, each pond will need a different amount of time for this process to work and it will also depend upon the size of the sterilizer and how slow or fast the water flows through it. Slower is better. I would estimate that a pond with the right sized sterilizer will have its green water cleared up within a week.



Just a quick conclusion to say thank you for reading and to state how important it is to follow the suggestions I have provided in this article.

Two of the things I feel that are most important when trying to control algae in an outdoor pond is to keep up regular maintenance of the pond (try spending some time on it every weekend) and to get yourself some plants that float on top of the water. This will not only help to reduce algae, but it will add cover for fish, so they feel safe and it just adds a nice ambiance to the feel of the pond.

Good luck!


Related Aquariums at Home Articles

Should You Use a UV Sterilizer in an Aquarium?

Why are Koi So Popular?

Can Koi Live in a Fish Tank?

How Small Can a Koi Pond Be? (Is Bigger Better?)