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Understanding The Workings & Biology Inside Any Pond Bio Filter ... The Nitrification Cycle

The waste products from the fish in their initial form are mainly expressed as ammonia which is poisonous in even small quantities and especially so under certain water conditions of high pH which will be discussed later. In a mature pond a class of bacteria in the biofilter removes the ammonia almost as soon as it is formed. The ammonia is converted into other nitrogen chemicals called nitrites.

These chemicals are less polluting and toxic than either ammonia or nitrites both of which are poisonous. The bacteria are called nitrosomonas and nitrobacter and other complementary types but let's just consider these 2 and ignore the rest as not being important for us non-biologists.

  • These chemical processes are occurring all the time and will not stop unless the biofilter system stops working or the source of nitrogen is removed ie no fish in the pond, or the water becomes devoid of oxygen.

  • The health of the pond is determined by this relentless ongoing conversion of ammonia to nitrates. It must NOT stop for the sake of your fish.

  • If pond water is changed frequently then to some extent the poison levels are controlled. The more fish in the pond then the more the waste products to get rid of. Koi produce 3 times more waste than goldfish of the same size - because they eat 3 times more.

The food used has a significant impact upon the amount of waste produced. Try to avoid the use of foods with high ash contents - this indicates low quality raw materials have been used in the food manufacture which result in water pollution levels being higher than necessary.

In a natural stream or lake fish concentrations are generally low. Waste products are converted to relatively harmless nitrates by naturally occurring bacteria. Rocks, submerged trees, plant roots, etc all help to purify the waste products from the fish by providing holding points for the bacteria to live on. The more bacteria there are the better the purification.

The bacteria need oxygen to survive and they get this from the circulating water - oxygen is absorbed by the water due to the action of waves, waterfalls and wind.

In a garden pond a biofilter is installed to make up for the unnatural conditions prevailing. A biofilter is designed to allow large concentrations of bacteria to operate effectively in a small volume within a garden pond environment.

Correctly specified and installed pond bio-filters create healthy environments in which fish can live for many years and grow to their full potential.

In small ponds the filter can be installed directly in the pond. For larger ponds the filter needs to be installed outside of the pond. The pump connected to a biofilter must run 24 hours every day otherwise the bacteria will die from lack of oxygen contained in the circulating water. It is a good idea to have a waterfall and a fountain to increase the oxygen content of the water.

It is difficult to add too much oxygen to a pond.

Should the bacteria in your pond filter die it will take approximately 5 more weeks for them to regain a population close to their previous levels.

What is happening inside a biofilter? ... thanks to The Pond Professor for this article

I created this picture below to help explain what happens inside a biofilter.

workings of a pond bio-filter

Imagine the red balls are ammonia excreted by the fish. Also imagine water is flowing over the biomedia from left to right.

You can see lots of red ammonia molecules are flowing into the filter although they are dissolved in the water in reality. Look carefully and you will see dark grey Alfagrog biomedia underneath the coloured balls. Look even more carefully and you might see the oxygen bubbles also dissolved in the water.

The ammonia mixed with oxygen in the water is continuously flowing across the biomedia. The yellow bacteria can be seen sitting on the surface of the biomedia just waiting for ammonia (this is the food for bacteria) and oxygen to come along.

At the point where the water meets the biomedia (i.e. where the bacteria live) the red ammonia molecules are broken down (oxidised is the correct term) by the bacteria which keep taking deep breaths of oxygen in order to complete the conversion of ammonia and nitrites to those blue molecules that you see starting to appear. These blue molecules are nitrates. You do not need to be a chemist to understand the next line .

NH3 is converted by one type of bacteria to become NO2 this is converted by second and different type of bacteria to become NO3

All I want you to take notice of is that ammonia has no oxygen, nitrite has 2 atoms of oxygen and nitrate has 3 atoms of oxygen in its molecule. If the water did not have oxygen dissolved in it then this conversion could not happen.

Notice the mixture of pretty colours in the middle of the picture this is where the conversion and intermingling action is taking place. You can possibly imagine lots of things are happening at the same time in this region ... because in fact there is a great deal of chemistry taking place.

The nitrates become the fertiliser for the plants and stay in the water until consumed by plants or pumped out of the pond with a water change.

You will notice that there are still a few red ball bits of ammonia left because the bacteria did not manage to get to them first time around circuit these get eaten next time or maybe even the next.

Just after feeding fish the red balls increase by an enormous amount and as such you will nearly always measure ammonia in the water shortly after feeding. However if the biofilter is mature and working well this is rapidly brought under control.

I have just described in simple terms what is called the nitrification cycle that occurs in all ponds and natural waterways.


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