a garden with a pond and a waterfall.

Many people find backyard koi ponds visually appealing because they are enchanting water features that can feel like private oases. However, maintaining a pond and ensuring the health and safety of its fish and aquatic plant life takes a lot of responsibility and knowledge. This is especially true when it comes to the nitrogen cycle.

Nitrogen is everywhere. Its molecules include ammonia, nitrate, nitrite, and other compounds essential for making plant material. Nitrogen is easy to come by, but hand-made water features like backyard ponds don’t have enough.

Keeping the water in your koi pond clear with enough nitrogen compounds to sustain aquatic life depends on the nitrogen cycle. Below is a guide detailing everything you should know about this process.

What Is Nitrogen Pond Cycling?

The nitrogen cycle is a biological process that transforms ammonia (NH3) into nitrite (NO2) and then into nitrate (NO3). The cycle is one of the essential building blocks of all life, and it isn’t entirely linear. The nitrogen cycle could start at any point and move backward or forward.

Many koi pond owners worry about the nitrogen compounds in their ponds and unintentionally alter their water chemistry incorrectly. Their efforts to create a clean water feature for their fish and plants could disrupt the pond’s ecosystem if done without knowing about nitrogen cycling.

Nitrogen pond cycling is an essential process that uses beneficial bacteria to break down toxic compounds in fish ponds. Large bodies of water usually undergo pond cycling naturally, but manufactured ponds need to facilitate the process using a biological filtration system.

The Nitrogen Cycling Process

a water fountain surrounded by rocks and a fish pond.

The pond cycling process involves circulating water in the pond using a pump and filter system to ensure that the water has the correct level of beneficial bacteria. Once the optimal level is present, biological filtration, or the nitrogen cycle, can occur.

Dead organisms and fish waste in the water contain ammonia. During the filtration process, the ammonia becomes nitrate because good bacteria feed on ammonia and produce nitrate as waste. Nitrate becomes a natural fertilizer for plants in the pond, which then become food for koi fish.

Koi fish owners often assume they need to cycle their pond after adding fish, but this isn’t the case. You want to initiate nitrogen cycling before putting fish into the water because it could already have many bacteria and proteins that could kill them due to excessive ammonia levels.

Natural Factors That Contribute to the Nitrogen Cycle in Ponds

Air, Rain, and Lightning

More than three-quarters of the atmosphere consists of nitrogen gas. Plants and animals cannot use this gas for food, but it enters ponds via rainwater, where it turns into nitrates, a consumable form of nitrogen.

Lightning can also introduce nitrates to a pond through atmospheric fixation. Nitrogen gas breaks up during lightning storms, and it combines with oxygen-forming nitrogen oxide. This compound dissolves in rainwater, which carries the nitrates to ponds. 

Aquatic Plants

Plants and algae in ponds absorb nitrate as fertilizer and expel nitrogen gas. The process is called denitrification, and it effectively removes nitrate from koi ponds.

Dead Organic Matter and Animal Waste

Organic materials in ponds may include leaves, insects, and lawn clippings. They release ammonia into the water as they break down, which initiates the denitrification process. You can regulate how much ammonia your pond produces with a skimmer filtration system. It collects organic debris in a basket you can remove and clean.


Circulating the water in your pond using a submersible pump or waterfall increases the water’s oxygen levels, which is necessary for nitrification. The surface area of rocks and gravel in the water feature also allows nitrification by providing the perfect breeding grounds for good bacteria. Oxygen and beneficial bacteria help ammonia become the consumable form of nitrate.

Why Is the Nitrogen Cycle Necessary for Koi Ponds?

a fish pond with rocks and plants surrounding it.

Koi ponds that fail to undergo the pond cycling process are a problem for several reasons. First, the nitrate that ammonia produces is a must-have food for pond plants. Aquatic plants cannot grow without the nitrogen compound, which means less food for koi fish and other organisms to eat.

Another reason for nitrogen cycling is eliminating toxins in the water that could kill the fish. Even the chemicals the cycling process produces naturally can become toxic in concentrated forms.

Nitrate and ammonia must remain within reasonable levels, 20 to 60 parts per million for nitrate. If the nitrate level reaches 80 parts per million, you will have to change the pond’s water. Anything above 120 parts per million could kill the fish.

Pond Cycling Supplies

Before fixing nitrogen compound levels in your pond, you must first gather the right supplies. You will need a filter with a high-quality submersible pump for nitrogen cycling. The pump keeps the water in the pond circulating to achieve the appropriate level of good bacteria.

Once beneficial bacteria are at the right level, the biological filtration will begin to balance everything out and make the pond more hospitable to your koi fish. You could also buy good bacteria and put them into the pond by hand to help the nitrogen cycle occur faster.

Keep Your Koi Fish Healthy with Services from Amen Corner Ponds

If you are ready to install a koi pond or need assistance maintaining an existing waterscape with fish and aquatic plants, turn to Amen Corner Ponds. We proudly serve residents throughout North August, SC, as a certified aquascape contractor with years of experience. Our team can install new ponds, perform routine cleaning and maintenance services, and repair and renovate existing water features.

Our pond experts fully understand the nitrogen cycle and use that knowledge to ensure that our ponds have clear water with the appropriate chemical compound levels. Request a consultation today!

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