Help! My koi pond is green! How to Quickly Fix a Green Koi Pond

A green koi pond in a garden.

Congratulations! Your koi pond is green because your nitrogen cycle is working. This is a great sign of a healthy pond. Keep in mind that green ponds, caused by algae overgrowth, tend to annoy humans more than fish, but there are levels of algae that can hurt your fish. Learn the causes of green koi ponds and how to fix a green koi pond.

A green koi pond in a garden. Koi pond is green

Why is my koi pond green?

Your koi pond is green due to a high level of cellular algae in the pond. Algae is a single-cell, photosynthesizing organism that lives in all bodies of water. When there is food for the algae, commonly in the form of nitrate or phosphate in koi ponds, the algae will grow and replicate, causing the water to resemble pea soup.

How do I fix a green koi pond?

There are a few ways to go about fixing your green koi pond. Long-term correction will require a multi-factor approach, but will help the most for the lifetime of your pond.

Add a UV Light

A UV light will work by zapping and killing all of the roaming green algae cells. However, this will turn your pond from green to brown, tan or gray. It only works when it comes into contact with the water containing the algae, so ensure that it is properly plumbed in. UV lights will not affect your pond in any other way than killing algae. Learn more about UV lights for koi ponds here.

Add in a fine particulate filter

Once your algae is dead, you will need a fine filter to remove it. You can do this with some fine floss filter media, but plan on cleaning it every 30 to 60 minutes after first adding. It will get clogged VERY quickly. Or you can go one step higher and plumb in a sieve filter. This filter has a very fine mesh screen that will remove the tiny dead algae cells. One step beyond that is a drum filter. These can get very expensive, but are essentially self-cleaning sieve filters. They will significantly improve your water clarity.

a pond in a garden with rocks and water. koi pond is green

How to Prevent a Green Koi Pond

Preventing your koi pond from turning green is even better than treating it. In order to prevent your koi pond turning green, you will need to take care of your algae food source. This primarily requires you to keep the nitrate levels, and phosphate if you have them, down as low as possible. This will prevent your algae colonies from getting out of control if they have unlimited food. Your nitrate levels will depend on your filtration, how many fish you have and how much protein they are being fed. Learn more about the nitrogen cycle and appropriate feeding for pond fish.

My koi pond is green, but it doesn’t bother me. Is that okay?

If your koi are not lethargic or gasping at the surface and come up for their regular meals, just as long as you are constantly adding aeration to your pond via a waterfall or aerator, yes, it is 100% okay for your koi pond to be green. Some people think that koi colors are actually more vibrant when the water is less than clear. We cannot confirm that, but it’s certainly something to consider.

Affects of Algae on Fish

If your koi pond is green, do not despair. Likely, your fish are just fine. As long as their is enough oxygen to go around, your fish should be just fine. However, the larger your fish are, the more oxygen they need. Your pond should have redundant aeration, such as a waterfall AND and air stone.

High algae levels can also affect the pH of your pond if your KH (carbonate alkalinity) is not high enough. During daylight hours, algae uses photosynthesis to grow and survive, however, when the sun goes down, this switches to cellular respiration, which produces carbon dioxide. Carbon dioxide is an acidifier and can bring down your pH if your KH does not buffer it out. You may never know this is affecting your pond unless you check your pH RIGHT BEFORE the sun rises. If you notice a big change in your pH, you need to decrease your algae load, add in more buffers or both.

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