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Although nitrate isn’t particularly harmful to koi, reducing nitrate levels can make a huge difference to water clarity
Koi produce large quantities of ammonia, not only as they digest food, but also as a result of normal activity. A 20-inch koi, in warm water, can excrete over 1,600 mg of ammonia per day. That is roughly a third of a teaspoonful of pure ammonia every day! Bear in mind that this is pure ammonia, not what is commonly sold as “Household Ammonia” which is typically less than 10% strength. A 20 inch koi excreting nearly a third of a teaspoonful of pure ammonia per day is the equivalent of adding around three teaspoonsful of household ammonia to your pond every day. Ammonia is toxic to fish, and so it must be removed from the pond water before it can build up to a level where it can cause them harm.
Removing ammonia
The traditional way to do this is with a biological filter. In these filters, the ammonia is first converted to nitrite and then to nitrate, which is relatively non-toxic to koi. This process is “aerobic” which means that it can only take place in well-aerated water.
Figure 1. How bacteria form a bio-film
Figure 1. Shows how bacteria form into colonies in a biological filter. The main bacteria populations are nitrosomonas, which convert ammonia to nitrite, and nitrobacter, which convert that nitrite into nitrate. To make this diagram more realistic, I have included a few extra bugs. It is usual for a few extra species of bacteria to also live in the biological filter. They do not take part in the nitrogen cycle, they are merely taking advantage of the free home. Together, all these bugs form into a “biofilm”. This is a semi-rigid structure that they create by sticking themselves together with what are called, polysaccharide Links. These links can be thought of as “sticky rubber bands” and they allow the bacteria to join together to form a colony, but with space in between each bug. These spaces and channels allow water to flow freely through the colony so that all inhabitants have access to the oxygen and nutrients that it contains.
Why remove nitrate?
If a biological filter can remove the ammonia, which is toxic, and replace it with nitrate, which is non-toxic, except at high levels, why not leave it at that? Although nitrate isn’t particularly toxic to fish, it is an excellent plant food. If it is not removed from the water, it will encourage the growth of algae, either in the form of green water or blanket weed. One way to remove nitrate from a pond is by means of regular weekly water changes. Each time we do a 10% water change, we are also removing 10% of the nitrate that is in the pond. In theory, when we refill, the concentration of nitrate in the pond will then be only 90% of the previous level.
The eagle eyed will already have spotted the flaw in this theory. In practice, it is common for tap water to contain a low level of nitrate. This means that when we top up a pond, we will have added slightly to the problem and the new level will not be quite as low as 90%, but for the sake of simplicity, let us pretend that we live in an ideal world and ignore this small discrepancy. A second water change the following week will reduce the level to 81%, and the third week will reduce it to 72.9%.
So, if we can get the level down to less than three quarters by just three water changes, is that the solution? Unfortunately not. Remember that 20-inch koi? It has been busy leaking a third of a teaspoonful of pure ammonia into the pond every day, and the bugs in the biological filter have been busy converting all that ammonia into nitrate. Clearly, as fast as water changes reduce nitrate, the biological filter is replacing it. Water changes are useful in reducing the nitrate problem but they are not the complete solution.
Vegetable benefits
Another way to reduce nitrate is by using a vegetable filter. This need not be anything more elaborate than a large shallow pond, full of plants. Typically, water returning from the filter to the pond flows into this vegetable filter and then overflows out of it, via a waterfall, into the main pond. Aquatic plants that grow rapidly take huge amounts of nitrate from the water as they grow. Any vigorously growing plant will do. Watercress is often used as a basis, and prettier species such as arum lilies or irises can be added to make the overall effect more pleasing to the eye.
An advantage of this system is that, in a suitable garden, it can blend into the surroundings. Disadvantages are; they have to be large and full of vigorous plants to be of much use. They can soon become untidy unless the koi keeper also has the time to become a water gardener. Positioning also has to be considered. Unless an additional pump is used, they have to overflow back to the pond by gravity, so they need to be higher than the pond itself. They can easily be added to an existing pond that is at ground level and where space is not at a premium, but the overall look and access for maintenance should be taken into account if the pond is above ground level or in a small garden. Oh, by the way, make sure that the neighbours will not object to the sound of a waterfall at night, or you may get a letter from your local council saying that a complaint has been made and suggesting that you turn off your filters at night.
Obligate and facultative
Anaerobic filters do not suffer from the disadvantage of size or noise. They seem to be very poorly understood in the koi community. Most likely this is because descriptions of them include words like obligate anaerobes and facultative anaerobes. These terms are easy to understand if you think of them in the following way: -
Obligate anaerobic bacteria are “obliged” to live anaerobically (where there is no oxygen). They are usually nasty, and love to live in sludge. Don’t allow sludge to build up in your pond or your filter and you will never have trouble with them (or even have to remember their name).
Facultative anaerobic bacteria have the “facility” to live anaerobically (where there is no oxygen) provided they can get a supply of oxygen by “stealing” it from some chemicals that happen to be floating by. We can make use of these guys. Nitrate has the chemical symbol NO3. This means that it is one atom of nitrogen with three atoms of oxygen stuck to it (N + O3). All we have to do is put a colony of facultative anaerobic bugs deep inside a hole where there is no oxygen but plenty of nitrate and they will get their oxygen by taking it from that nitrate as it floats by. If all three oxygen atoms are removed from the nitrate, all that will be left will be the nitrogen atom, and since nitrogen “prefers” to be a gas, it will bubble away into the atmosphere at the first chance it gets. All we have to do is give our little facultative bug friends the right media, and they will remove the nitrate for us. This media is called denitrifying media.
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Figure 2. Denitrifying media
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Nitrogen Cycle
Figure 2 shows how this media works. If biological media has microscopic holes or cracks in it, then the bugs living on the outside, and those just inside the hole, have easy access to water that is well oxygenated. Deeper inside the hole, the blue bugs will find that much of the oxygen in the water has already been used by the red bugs. Life will be harder but they will be able to survive because there will still be some oxygen left, and they will use the last of it. Both of our nitrogen cycle bugs (nitrosomonas and nitrobacter) can live in the red and blue areas and between them, they will convert ammonia into nitrate.
 At the end of the hole, however, living conditions for the mauve bugs are very different. There will be no oxygen left in the water that reaches them, but it will contain plenty of the nitrate that has been produced by their red and blue neighbours. The only bugs that can live here are those that can take their oxygen from this nitrate. In other words, we have made a perfect home for our little facultative anaerobic friends, and they will repay us by removing nitrate from our pond water, thus starving the algae of it’s food.
A problem with this system is that the water fed to this media must be very clean. Even tiny particles in the water can block the entrances to the holes in which the bacteria live. If the holes become blocked, not only will the mauve bugs not be able to remove any nitrate, but some of the red bugs, and all of the blue bugs, in figure 2, will also be sealed in by the blockage. These are the usual filter bugs, and if the reduction in available space results in the colony size of these two types being reduced, then not all the ammonia will be removed from the water before it returns to the pond. If a system containing denitrifying media is well set up and maintained, the water quality will be impeccable, if not, the water quality could be very poor.
Explosive Pressure
Bakki Showers and their clones are reputed to reduce nitrate. The assumption is, that this must be due to the porous structure of the media which will allow facultative anaerobic bugs to set up home as in figure 2. There is nothing in the physics or the chemistry that suggests to me that this is the complete story.
The basic design of these showers ensures that the water is splashing and forming into thin sheets as it tumbles over the media from the top to the bottom. This action forces oxygen to dissolve to the point where the water cannot hold any more, (saturation point). This pummelling of individual drops of water also forces out any other dissolved gas.
 Not convinced? Consider this; if the top is removed from a bottle of lemonade, it will eventually go flat as the carbon dioxide gas in it bubbles slowly to the surface and away into the atmosphere, but if the bottle is shaken vigorously, the carbon dioxide is released very much more quickly. As the lemonade is shaken, the gas comes out of solution so fast that it is almost explosive.
Ammonia is also a gas. It will dissolve in water but it “prefers” to return to being a gas. When water, containing ammonia, is shaken about violently, the ammonia comes straight out of that water. This is exactly what happens in Bakki Showers. A great deal of ammonia is removed by the pummelling action as the water tumbles over the media. Removing ammonia before it goes through the nitrogen cycle means that less will remain to be turned into nitrate by biological filtration, leading to reduced levels of nitrate in the pond.
Anoxic fitration
Anoxic filtration is a system that has been developed and trialled over many years by Dr. Kevin Novac Ph.D. Water clarity in pictures of ponds that are using it is amazing. Not only will this system remove ammonia directly from pond water before it can be turned into nitrate, but it also has areas where friendly facultative bugs can live and remove any nitrate that has been produced by nitrogen cycle bugs elsewhere in the pond. The basis of the system is a 24 inch-deep pond, full of what are called Biocenosis Baskets. These baskets are nothing more complicated than a planting basket full of Kitty Litter with a volcanic material called Laterite poured into a depression in the centre. Some of the baskets must be planted, but all baskets may be planted, according to taste. Dr. Novak is an American, and Kitty Litter is a common American brand name. In the UK, an equivalent is Fuller’s Earth.
The way these baskets work is too complicated to explain in detail but relies on the fact that, according to Dr. Novak’s research, although nitrate is an excellent plant food, ammonia is a better one. Given a choice, the plants in these baskets will take ammonia direct from pond water, in preference to nitrate. Amazing but true! Ammonia (NH4+) also has an electrical charge. Opposite electrical charges within the basket draw ammonia in. Facultative anaerobic bugs ensure that any nitrate in the water is stripped of it’s oxygen and the nitrogen bubbles away in the same way as in denitrifying media in figure 2. And that’s the simple explanation, the full details are mind numbing!
Less is good
There are other ways to reduce nitrate, but whichever method is used, there are no disadvantages in reducing it. Less nitrate means less algae without having to resort to using chemicals to poison it.
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