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Good water guide     (Good water guide)


I wrote the following articles for Koi Carp Magazine.
Therefore they own the copyright but the Editor has given permission for them to be republished here.
Thank you, Karen.

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Aimed primarily at beginners to the hobby, this series of articles will take you step by step through the process of understanding how a good koi pond works.

Part 1: The necessity for biological filtration

With so many different types of pond filters that are either available on the market or that have been built by koi-keepers it might seem to those new to koi keeping that they all work on different principles. As this series progresses, the differences between various types of ponds and filtration systems will be explained, as will the elements that make up those systems but, since this is a beginners’ series, it would be best to start with the most basic question of all: “What are the essential features of a koi pond?”

The answer is simple; apart from the pond itself, a biological filter and a pump to circulate water between the pond and the filter are the only two essential features. Figure 1 shows how these elements work together to biologically filter a koi pond. The water flow pattern is not ideal, there would be many disadvantages to such a simple system and it is these disadvantages that are addressed by the extra elements in more complex filter designs. But the diagram will serve well enough to explain how biological filtration works, so let’s begin with an explanation of why biological filtration is needed, how it may be achieved and then develop an ideal pond from there.

Beginner basic

Figure 1:  Basic biological filtration - this pond has many disadvantages which will each be addressed as this series continues but the diagram is all that is essential for a koi pond

Why biological filtration is necessary
Koi need protein in their diet - protein contains substances called amino acids and amino acids are compounds based on ammonia.  After all we have been told about the toxic effects of ammonia, it may seem hard to believe that koi actually need foods that contain ammonia but this is a fact of life for them. As protein is digested and then metabolised, the ammonia in it is released as a waste product and is excreted into the pond, mainly through the gills. There are two forms of ammonia in a koi pond and the difference will be explained later in the series but, for simplicity, I will just refer to “ammonia” for the moment.  If it is allowed to build up in the pond, it will reach harmful or even toxic levels.  It is for this reason that biological filtration is a necessity.

How biological filtration works – the nitrogen cycle
The biological filter, or bio-filter, is where ammonia is removed from the koi environment and is simply a convenient place for two naturally occurring species of bacteria to live. These bacteria, (nitrosomonas and nitrobacter), are called nitrifying bacteria because they depend on nitrogen for their energy source in what is called the nitrogen cycle.

The nitrogen cycle isn’t something that is specific to koi ponds or even ponds in general, different versions of the nitrogen cycle are common throughout nature, not just in water, but in soil and even in the atmosphere, our two nitrifying bugs are very successful and can thrive in a wide range of environments, above and below water. Actually, these two bacteria have a few cousins, with equally complicated names that will sometimes join them in these processes but, for a simple description of the nitrogen cycle as it applies to koi ponds, they can be ignored until later in the series.

When nitrifying bacteria enter a pond, they colonise any wet surface including the floor, walls and even inside pipe-work. In fact, they prefer the inside of pipe-work to open surfaces because they are photophobic, which means that they don’t like light. Light doesn’t kill them, they can adapt to it but they prefer dark places and will multiply better without light. This gives us the first clue to what makes a successful bio-filter - it should be kept dark.

Our two favourite nitrifying bugs can swim!  Just about. They have little tails that can whip about and propel themselves through water. The ammonia bug (nitrosomonas) can propel itself quite well and the nitrite bug (nitrobacter) will propel itself occasionally.  They can also take part in the nitrogen cycle whilst on the move but are more efficient when they are settled onto a solid surface.

This makes a second requirement for a good bio-filter - it should have a solid surface where the bugs can settle and form what is known as a biofilm.  This is where they attach themselves to a surface and to each other, making a thin layer all over the surface that is very hard to dislodge.  Figure 2 shows how bacteria form into a biofilm by attaching themselves to the surface and to each other. Usually, other bacteria join in to the biofilm as well in addition to the two that we want. They take no part in the nitrogen cycle; they are simply taking advantage of a suitable home and are shown only to make the diagram more realistic.

Early bio-filters contained media such as gravel, pieces of chopped up pipe or hair rollers. Once a biofilm has formed, converting ammonia to nitrate by means of the nitrogen cycle becomes very efficient.  These bacteria do not remove ammonia from the koi pond as a favour to koi keepers. Their sole purpose in life is to absorb these two chemicals that we would refer to as pollutants, and to use them in order to grow and to reproduce.


Figure 2:  How bacteria form into a biofilm

This is a third clue on the path to a good bio-filter. Bacteria need to grow and to reproduce so we must provide them with the space to do so or their colony size will be small and the rate at which ammonia is removed from the water will slow right down. This is because they have very little use for ammonia, or nitrite, other than as an energy source that allows them to grow and then to divide into two separate bugs that will absorb more ammonia or nitrite in order to grow and split into even more bugs.

A fourth clue involves complex bio-chemical equations far beyond the scope of this part of the series but it is well known that nitrifying bugs need oxygen, They cannot remove ammonia or nitrite from water without it and, in removing it, they use  just over four times as much oxygen as the original amount of ammonia. They won’t die if their oxygen supply is interrupted, not in the short term at any rate, but they will go into a sort of dormant stage until the level has risen sufficiently to allow them to continue.  If oxygen is available but it is only at a reduced level, bacteria are far better than koi at scavenging what little oxygen there is from the water. The minimum acceptable level for dissolved oxygen for koi is 6 mg/L. If the level was to fall below about 3 mg/L, koi will soon die but nitrifying bugs can still function at levels down to about 1.5 mg/L before becoming dormant.

Drawing those points together, if the media in which nitrifying bugs are living, isn’t sufficiently oxygenated, nitrification will slow down or stop altogether.  Bugs near to the edges of the media will still be able to take oxygen from the pond water, and will do so at an alarming rate. In doing this, they may deplete oxygen below the point at which koi will die.  Bio-filters need to be well aerated!

Requirements for a biological filter

Beginner pipe
Figure 3: Chopped up pipe is mainly hollow


Beginner gravel02
Figure 4: Smooth, rough and very rough gravel


Beginner Canterbury Spar03

Figure 5: Canterbury Spar

Taking stock of these requirements for a good bio-filter, we need a dark place with solid surfaces for the bugs to form a biofilm with plenty of room for them to grow and multiply.

One of the early ways to do this was to have a container that was full of chopped up pieces of pipe, flocor or even hair rollers. There is nothing wrong with these types of media, except that they are mainly hollow and bugs prefer surfaces that they can attach themselves to.

Figure 3 shows an example of chopped up pipe media.  A bacterial colony can grow on the inside and outside surfaces but the hollow spaces inside are of no use to them.  This means that due to the limited surface area, the only way to create space for a large colony of bacteria would be to have lots of this type of material and this would need a very large bio-filter bay or chamber to house it all.

In any given size of bio-filter, gravel type filter beds allow more surface area for bugs to use, so this was another popular media because bio-filters could be physically smaller. Figure 4 shows a diagrammatic representation of the characteristics of different types of gravel, smooth, rough and very rough.

Figure 5 shows what Canterbury Spar looks like close up. Gravels and similar media are rarely perfectly round but, as the diagram illustrates, the rougher they are, the more space there is on each piece for bacteria to grow. This allows the overall space taken up by the filter media bed to be smaller without reducing the available space for the biofilm.

There is nothing wrong with big filters except the space they take up, so unless space is a problem, it isn’t necessary to replace an existing bio-filter that is performing well just because it is using an “old fashioned” media.  However, I wouldn’t recommend these types of media for a brand new bio-filter because there are newer types of media that will allow more surface area on which the bugs can grow, but in a much smaller overall volume.

What media?

Beginner jap-mat
Figure 6: Water flow through Japanese Matting

Nitrifying bugs are neither house-proud nor neighbourhood snobs, they don’t care where they live, and are just as happy on one surface as another, but different types of media offer the pond owner the opportunity to have filters with different characteristics such as being physically small or particularly easy to keep clean. These individual characteristics of different bio-filters will be discussed later in the series, but to stay with general principles for the moment, what should we be looking for in filter media? Small physical size is optional but desirable. Also there should be plenty of surface area on which the bacterial colony can grow. It is vital that it allows free movement of aerated water through it.

There are many examples of suitable media that fit these requirements but Jap-mat is possibly the easiest to look at and see at a glance what is happening.  It isn’t a new media but, in figure 6, it is included as an example to illustrate how aerated water can easily flow through the channels between individual sheets and percolate into the media where bugs that are involved in the nitrogen cycle are colonising the individual strands.

So far, in this overview of biological filtration principles, only conversion of ammonia to nitrite and then to nitrate by means of the nitrogen cycle has been covered. Although the resulting nitrate is far less toxic to koi than ammonia or nitrite, if it is allowed to build up, it will not only encourage algae growth but it is now being recognised that it can affect fish growth and colour development.  As will be covered in part two of this series, the usual way to reduce nitrate levels is by means of water changes, but there are different types of media, known as denitrifying media, that can allow the development of another type of bacterial colony that will remove nitrate from our ponds.  The way they do this will be covered later, as will the drawbacks of a pond that only has a basic biological filter and nothing else.

The pond in Figure 1 will adequately remove ammonia, which is the primary pollutant in ponds but it needs much more before it could be developed into an ideal koi pond and it is these improvements which will be discussed in part 2 of this series.


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