Garden Pond Filter Interaction between Nitrifying Bacteria and Oxygen

A Garden Pond Filter Allows The Nitrogen Cycle To Take Place

Any garden pond containing Koi, goldfish, pond plants and other aquatic pond life such as frogs, snails, salamanders etc will have plenty of biological and metabolic processes taking place within it. The average backyard garden pond will contain more aquatic life than it should have. As this pond life, particularly Koi and other pond fish eat they release metabolic waste products into the pond water. The quality of the pond water is rapidly compromised, particularly by ammonia (NH3), a lethal chemical in even relatively small amounts.

Here is a short story, bear with me on this. As a child I used to keep goldfish in a goldfish bowl. Mum, would buy a goldfish for me from the local pet store and I would place my new best friend in the goldfish bowl and sit and watch it for hours. I used to especially enjoy watching it feed on the fish food I fed it. The one thing however that I never understood was why after only a few weeks I would always wake up to find my goldfish floating on the water surface, obviously dead. I must have had about 10 goldfish before I finally lost interest in keeping fish.

It wasn't until years later when I became interested in water gardening that I finally came to realize why my poor old goldfish always died. The reason was down to ammonia poisoning. A goldfish kept in a goldfish bowl, if you think about lives in a small volume of water, where it eats and goes to the toilet. The metabolic waste excreted into the goldfish bowl contains ammonia, which over a period of a few weeks reaches a level where it poisons the poor thing.

In a garden pond, even though there is a larger volume of water than in a goldfish bowl the level of ammonia build up will increase significantly, particularly if you have plenty of Koi, which are voracious eaters.

The only way of preventing ammonia poisoning is to remove ammonia as soon as it is released into the pond water. The most effective method is to use a biological pond filter; biofilter for short.

A biofilter allows the Nitrogen Cycle to take place in your garden pond. This process is essential for the breakdown of ammonia, contained in pond water into nitrates. Strictly speaking it isn't the biofilter that does this it is Nitrosomonas and Nitrobacter beneficial bacteria that live on the pond filter media, inside the filter chamber. These nitrifying bacteria use nitrogen as a food source and readily convert ammonia (NH3) which contains nitrogen.

The Nitrogen Cycle and the interaction between nitrifying bacteria and oxygen in a garden pond filter is a constant process that continues until you switch your biofilter off or the biofilter stops working. A word of warning: DO NOT switch the pond filter off, as after only a few hours the nitrifying bacteria, responsible for breaking down toxic ammonia will die and it will take approximately 5 weeks for the colony to build back up again.

Factors Affecting Ammonia Levels In garden Ponds

If you ever visit a Koi show you will see koi owners constantly changing the water to try to remove as much ammonia as possible. A koi on average will eat 3 times as much as an equivalent sized goldfish and are capable of producing a lot of solid metabolic waste.
The quality of fish food or Koi food fed will have severe implications upon the levels of ammonia produced. Inferior fish food, containing high levels of ash will result in more metabolic waste, as less of the goodness can be absorbed by the Koi and is subsequently excreted into the pond water.
Pond sludge build up at the bottom of the pond from leaves, grass and uneaten fish food falling to the pond floor starts to rot giving off ammonia. Try to get into the habit of removing leaves and other floating debris from the pond surface as soon as you see it with a net. A pond skimmer is an excellent way to mechanically remove organic matter on an ongoing basis.
Natural lakes and ponds, unlike a much smaller garden pond have vast quantities of water and constant surface movement, due to waves, tides, wind etc. This combined with a large surface area for oxygen to dissolve from the atmosphere into the lake or pond creates an ideal natural environment for ammonia breakdown. It is the same nitrifying bacteria (Nitrosomonas and Nitrobacter) that convert ammonia. They colonize rocks, plants and any available surface. This provides for an extremely large surface area for them to live on. In addition the volume of water and number of fish capable of producing ammonia per cubic foot of water is a fraction of that in the average garden pond
Oxygen levels have a major impact upon the level of ammonia that can be converted, as the nitrification process (nitrogen cycle) requires copious amounts of oxygen. In any pond environment ensure that there is a plentiful supply of oxygen. Adding a waterfall, fountain or pond air pump will help. You can never have too much oxygen in your garden pond.

How do Nitrifying bacteria And Oxygen Interact In A Garden Pond Filter?

Take a look at the diagram below, courtesy of Tony Roocroft (The Pond Professor), which shows the interaction between ammonia, oxygen and beneficial nitrifying bacteria in what is known as the Nitrogen Cycle or Nitrification Process.

In this diagram the pond water flows from the left, containing ammonia (red balls) and oxygen (light grey balls). The water passes over the pond filter media (dark grey background) containing the nitrifying bacteria (yellow balls) that sit there waiting for ammonia.

The nitrifying bacteria Nitrosomonas, oxidizes (breaks down) the red ammonia molecules into nitrite (purple color). This is then converted into nitrate (light blue balls) by Nitrobacter.

The nitrates produced remain in the pond water and are a valuable source of fertilizer for aquatic plants. Unfortunately excess nitrates are also a valuable food source for pond algae, which needs to be controlled using a UV sterilizer (UV clarifier).

How Can I Improve My Garden Pond Filter Performance?

Change the pond filter media - If you have a small to medium sized garden pond and use either a box type gravity filter or compact pressurized pond filter then you can significantly improve its ability to oxidize ammonia by simply replacing the existing plastic pond filter media with Fishmate Supra biofilter media. This low cost, porous biomedia has over 100 times the surface area of plastic coils.
Add a waterfall, fountain or pond air pump to your garden pond. The oxidization of ammonia by Nitrosomonas and Nitrobacter requires large amounts of oxygen. Any pond environment will benefit from as much oxygen as possible.
Remove any excess pond debris such as leaves, grass, twigs etc before it has a chance to settle on the pond floor, where it decays and produces ammonia or enters the pond filter causing blockages and encouraging heterotrophic bacteria colonies to develop. Heterotrophic bacteria pollute pond water and causes fish fatalities.