Koi Pond Algae Control

Is your koi pond turning color from clear to green? Or are your koi fish having difficulty breathing even in what looks like an algae-free pond? Perhaps it is the algae. Your koi fish will die fast if you don’t take the correct control measures. Sudden and mysterious koi deaths could also mean algae in your koi pond.

There are many ways to control algae and restore the oxygen levels in the water. The most natural way to control algae in your koi pond is to introduce algae eaters such as large-sized turtles, plecos, and mystery snails. Grow lilies and lotus plants to block sunlight from encouraging algal growth.

Although koi fish is among the best algae eaters, excess algae can overwhelm them. Physically removing the algae can also be labor intensive. Where the algae are becoming a menace to your koi fish, seek algae treatments like peroxides and ultraviolet sterilizers. 

I will discuss them in detail below. You will also learn the signs of algae in the koi pond. Let’s get started.

Signs of algae in koi pond

Here are some signs that your koi pond has algae:

1. Pond water turning color from clear to green

Suspended algae cause the greenish water you see in your koi pond. They are single-celled algae so tiny that they can escape even the most exemplary filter. The algae can double in size within 24 hours, turning your koi pond water green, cloudy, and murky. 

Overfeeding koi fish can result in waste and leftover decomposition in the koi pond. Thus, the imbalance in water chemistry accelerates algae growth in your tank. These can suffocate your koi fish and leave them gasping for air at the top of the pond.

As you will learn in the next section, removing food leftovers in the fish tank is essential to controlling algae in koi ponds. It curbs decomposition and lowers the rate at which the pond water turns green. You will learn more ways to correct the situation later.

2. String algae’s green filaments

String algae look like hair strands. They are filamentous, and they increase in the presence of sunlight. They use oxygen and sunlight for photosynthesis. Therefore, if you don’t manage string algae correctly, it can use all the oxygen in the water and suffocate your koi fish.

As time passes, string algae tangle together to form a large algae mass that looks so bad. Unlike suspended single-celled algae, mature string algae are easy to see and remove. You can use physical, chemical, or electrical means to control string algae.

Learn more about koi pond algae control below. 

How to control algae in koi ponds

Here are some ways I have used many times to control algae in my koi pond:

1. Biological algae control

The best way to control algae in your koi pond is to use biological mechanisms like plant and algae-eaters. 

Remember that algae rely on sunlight for photosynthesis. Therefore, growing floating plants like lilies and lotus on top of your koi pond not only provides shade to the fish but also discourages algal growth.

Another biological algae control in koi ponds is using algae-eaters such as mystery snails and a common pleco. Mystery snails are popularly known as pond cleaners. They will feed on the algae and keep your pond sparkling clean. 

However, I recommend incorporating other koi pond algae control measures like physical removal before adding new pets to your pond.

2. Physical algae removal

Sometimes, the pond may be heavily infested with mature, tough algae that koi fish, common pleco, or mystery snails can not eat. At that stage, the oxygen levels in the koi pond could also be insufficient. In that case, start by removing the algae physically.

You can remove the algae by hand or using a pond skimmer net. Although physical algae removal is cost-effective, it could leave behind algae cells that would bloom when conditions allow. Besides, removing algae manually might not restore pond water clarity.

3. Algae treatments

The prospect of adding algae treatments in koi ponds terrifies most fish owners. However, there are viable algae treatments that will not harm your fish. For example, granular-based peroxides are safe for koi fish if you apply them at the recommended dosages.

Like the common hydrogen peroxides, granular-based peroxide oxidizes the algae and releases oxygen safe for your koi fish. Although it is costly, granular-based peroxide does not leave dangerous toxins in your koi tank. 

However, after using the treatment, you will have to sieve the dead algae filaments and residues. Another safe alternative algae treatment for a koi pond is diquats. Use diquats to control algae for display koi fish and not on the koi fish meant for human consumption.

4. Removing food leftovers from the pond

Avoid overfeeding your koi fish. An adult koi fish should be satisfied with a three-minute meal. The fish is most likely to leave any excess food. Rotting leftovers may not increase algae in your pond directly, but do so by clogging and reducing the efficiency of pond filters.

Therefore, there are several ways to remove food leftovers. You don’t need to drain all your pond water to remove the food leftovers. Use a pond vacuum instead. It is an effective tool for draining the bottom sludge in your koi pond. Consider the Matala Power Cyclone pond vacuum.

5. Control koi pond algae using ultraviolet sterilizers

Algae will die if you expose them to extreme ultraviolet sterilization. UV radiation damages the algae cell walls, stops them from reproducing, and eventually kills the algae. 

Ultraviolet sterilizers are effective against suspended algae and string algae. Besides, they do not need a lot of maintenance. 

The TetraPond UVC-5 GreenFree UV Clarifiers will help you control algae in your koi pond and keep the water green-free. It is pretty easy to set up; it takes only 15 minutes. 

Once installed, you will notice the difference in your koi pond within three or four days. 


Texas A&M Agrilife Extension. (n.d). How to Control Filamentous Algae. A Diagnostics Tool for Pond Plants and Algae

Texas A&M Agrilife Extension. (n.d). Types of Algae

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