Simple nitrogen cycle | biofilter media & the nitrification process

The Nitrogen Cycle or Nitrification Process explained

In order for the simple nitrogen cycle to take place in your biological pond filter naturally occurring ammonia oxidizing bacteria need to be present on the biofilter media housed in the filter chamber. The higher the SSA (specific surface area) of the pond filter media the better as this ensures higher numbers of beneficial bacteria to carry out the process. A regular supply of nitrogen in the form of ammonia is required (provided by koi and other pond fish solid and liquid waste). A regular supply of oxygen is also needed; normally provided in a garden pond environment by an air bubbler, water fall or oxygenating plants.

The Nitrogen Cycle or Nitrification Process explainedHow does the simple nitrogen cycle take place in pond filter systems?

As the koi and goldfish in your fish pond feed they produce solid and liquid waste which contains a chemical compound known as ammonia (NH3) that is highly toxic and potentially lethal to your fish, unless removed using a Biofilter. The Biomedia in your pond filter is home to colonies of beneficial bacteria (ammonia oxidizing bacteria). Theses bacteria live on the pond filter media's surface. These naturally occurring bacteria, using a series of biological processes known as the Nitrogen cycle or Nitrification process convert the highly toxic ammonia to much less harmful nitrate (NO3), which acts as a fertilizer for the aquatic plants in your fish pond.

  • Nitrification process - Stage 1:  An 'ammonia powered' bacteria known as Nitrosomonas converts ammonia to Nitrite (NO2) which is less harmful than ammonia but will still poison your fish if sufficient quantities remain in the pond environment. On average your fish are capable of withstanding twice the level of nitrite in the water than ammonia (based on same volume of pond water).
  • Nitrification process - Stage 2: Bacteria known as Nitrobacter convert the nitrite produced during stage 1 into nitrates. Nitrates will remain in your garden fish pond indefinitely, unless there are pond plants present that use the nitrates as a valuable source of nutrients. Aquatic pond plants use up the nitrogen part of the nitrate to aid their growth.

Note: The beneficial bacteria in your koi pond biological filter chamber need a constant supply of oxygen (24/7/365) for the nitrogen cycle to take place. If oxygen isn't available then within several hours the beneficial bacteria will die resulting in ammonia levels increasing.

How long before a new bio filter beds in & the Nitrogen Cycle is working properly?

After installation of your fish pond filter it will take approx 6 weeks before optimum conditions have been reached for the beneficial bacteria to perform their ammonia oxidization duties. During the interim period the following MUST be observed:

  • Large amounts of pond water need to be replaced on a regular basis.
  • Don't overstock your fish pond, especially with gluttonous koi.
  • Don't over feed your Koi, they are hardy and can survive on insects and larvae that fall into the garden pond.

Many commercially sold pond filters are supplied as combo units, that include both a UV sterilizer and a biological filter. The reason being twofold: firstly a financial one in most cases and secondly because of the ease of installation. Larger koi ponds on the other hand will require Professional Biofilters due to the volumes of water and the levels of koi.

It is important to understand that the UV sterilizer (UV clarifier) plays it's part in the water purifying process by removing / preventing green water. It is important to Manage Algae Bloom (green water) because too much will starve the pond of oxygen and suffocate your koi and goldfish.