Inorganic wastes in Koi ponds - the stuff you can't see...

We've touched on inorganic wastes. Let's take a closer look.

Koi filter systems have to deal with inorganic wastes via the use of a bio filter. A clear understanding of the biological process is essential.

Manage the wastes and manage the Koi pond!

As we've seen in are Koi really swimming ammonia factories, inorganic wastes are dealt via a bacterial process in a cycle called the nitrogen cycle.

Let's examine this process in a little more detail. The nitrogen cycle relies on two useful types of bacteria for the conversion of ammonia produced by Koi to nitrates (plant food). The first stage is the conversion of ammonia to nitrite and this is performed by a bunch of ammonia munching bacteria called Nitrosomonas.

The second stage of the nitrogen cycle is the conversion of the nitrites just produced into nitrates and this process is facilitated through the help of friendly bacteria called Nitrobacter.

The obvious thing is that the more of these guys that hang around in our Koi ponds the better. They're the same bacteria that Mother Nature use for the same purpose. But Mother Nature has a much larger area for these bacteria to inhabit relative to the number of fish present.

Koi keepers get around this problem by using bio filters. In essence these are devices through which pond water flows and which contain some kind of bio media. Bio media is anything that our friendly bacteria can colonise on since they are not free swimming bacteria.

This means that surface area becomes all important. The more surface area available, the more bacteria you can fit into your biofilter.

Not all bio media is created equal. Some are far superior than others as we will see. Some have exceptionally high surface areas in relation to their volume that they occupy yet are unsuitable for bacterial colonisation as we'll see.

The Nitrogen cycle explained

As always the key to understanding what makes a good bio filter starts with understanding what the bacteria are actually doing.

Firstly we need to understand that Nitrosomonas and Nitrobacter, which collectively are called nitrifying bacteria are slow growing bacteria. When you're as small as a bacterium, this is not a great competitive advantage. By the time our useful little bug friends have made their presence felt we certainly don't want them to find that their potential living space has been taken over by other less useful bacteria.

The only way we can do this is to try and make the living space set aside for our wanted nitrifying bacteria is as inhospitable to other common bacteria as possible. This we can control by food! If there is no food for the bacteria we don't want, then they won't grow. We'll come back to this in more detail further on.
There are two key reactions that take place. Firstly we have the Nitrosomonas who reduce ammonia to nitrite via the following reaction:

55NH4++ 76 O2 + 109HCO3- C5H7O2N + 54NO2-+ 57H2O + 104H2CO3

This looks horrific but it is really easy to understand. What we can see that is important is that on the left hand side, we're using up 76 Oxygen (O2) molecules for every 55 molecules of ammonia (NH4+)converted. There is no oxygen present on the right hand side which means it's all been used up.

This means that we must have plenty of oxygen in the water for the bacteria to be able to complete this reaction. If they run out, the reaction cannot continue and they can't get the job done completely.

Also important to note is that the Nitrosomonas produce almost exactly the same quantity of nitrites (NO2-) as ammonia used up, 54 molecules in all.

The second stage of the nitrification process is performed by the Nitrobacter and goes as follows:

400NO2- + NH4+ + 4H2CO3 + HCO3- + 195 O2 C5H7O2N + 3H2O + 400 NO3-

Once more we can see that 195 molecules of oxygen (O2) are used in reducing 400 molecules of nitrite (NO2-) to 400 molecules of nitrate (NO3-). Again, the oxygen is important. If there is not enough, this reaction will also come to a grinding halt and in fact we're worse off since each molecule of oxygen reduces just over 2 molecules of nitrite (NO2-) to nitrate (NO3-).

Combing both these reactions and doing a little mathematics we come to the certain conclusion that to reduce 1 mg of ammonia into nitrates, we need to use up 4.3 mg of oxygen.

Oxygen is vital in Koi ponds and especially in bio filters

Oxygen is thus a critical element of the nitrification cycle, as odd as that may sound. Yet what happens in so many Koi pond bio filters? The only oxygen that is available to our hard working much appreciated bacterial buddies is that which is present as dissolved oxygen in the water. Once that is used up, no matter what size your bio filter is, the nitrification cycle comes to an abrupt and spectacular halt.

In fact, in oxygen deprived water, a condition which is known as an anaerobic one, you will not find many friendly bacteria at all. On the contrary you will find anaerobic bacteria growing and whilst these have their place in the greater scheme of things we don't want them in our Koi ponds. These bugs do play a role in the filtration process but the by products of their reactions can quickly poison Koi and lead to spectacular Koi losses.

Anaerobic conditions, unless they are rigidly controlled are not desirable in a Koi pond. They are a haven as a breeding ground for unwanted pathogens and disease causing bacteria. More on this in organic wastes.