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 16: Nitrate and the problems it causes
Before continuing to discuss the problems caused by nitrate in koi ponds, here is a quick recap on how what we call the nitrogen cycle isn’t really a complete cycle at all. Figure 1 shows a highly simplified representation of how the cycle would be completed if we grew sufficient plants in our ponds and figure 2 shows how the cycle is broken where plants are not grown in sufficient numbers. In the average koi pond, the nitrate that is produced as a result of bacterial action by the nitrite bug isn’t removed by plants; it just builds up in the pond water.
Nitrate plus phosphate equals algae
It is well known that nitrate in pond water has the effect of encouraging filamentous algae (blanket weed) and planktonic algae (green water). This effect is greatly increased by another pollutant in pond water that doesn’t get as much publicity as it deserves – phosphate.
Phosphorous is another element that is essential for life. It is necessary for such things as the teeth and bones of all animals and humans. The individual cells in our bodies and all other living bodies, including plants and bacteria, require it. Even DNA couldn’t exist without phosphorous.
It should, therefore, be no surprise that phosphorous is added to koi food in the form of phosphate. Koi use the phosphate that they need and excrete the excess into the water. This phosphate, in combination with nitrate from the (incomplete) nitrogen cycle in biofilters makes an excellent plant food. If there are no plants in a koi pond to use it, this nitrate-phosphate combination becomes a powerful fertiliser for algae.
Nitrate spoils koi
There is a second problem. It is often said by those who breed or grow on koi that nitrate has a detrimental effect on koi growth and skin quality but, until recently, there were never any concrete facts or research that we could point to in order to back up this belief. In fact, although it doesn’t seem to be widely known, the research into the negative effects of nitrate on koi has already been done.
After reading through some old and almost forgotten articles, Kevin Novak PhD discovered that, around 15 years ago, Takayuki Izeki, wrote about his research into the effect of nitrate on the shiroji (white) of kohaku. He found that the maximum acceptable level of nitrate for koi keeping should be 15 mg/L, saying: "anything more than this and the skin gradually begins to deteriorate but will improve again if the concentration is reduced."
He also said that if the pond nitrate level could be kept lower than 5 mg/L, "the skin becomes so white it virtually shines". His research found that the negative effect of nitrate was caused by it reducing the koi’s ability to discharge metabolic toxins from its body and that this resulted in these metabolic wastes building up in the skin.
One way to help solve the nitrate problem is by regular weekly water changes. Each time we do a 10% water change we are also removing 10% of the nitrate in the pond but, while there are fish in the pond, water changes alone won’t get nitrate down to such low levels. We need something more and bugs, as usual, can help us.
In this series, wherever I have been able, I have tried to keep the discussion of the principles of ponds, filters and equipment as simple as is possible and avoid using long scientific names. Sometimes this can’t be avoided especially when describing the principles behind some of the more advanced filtration techniques. There is a group of bugs that have the frightening description “facultative anaerobic chemo-litho autotrophic bacteria”. Although these bugs have lifestyles that are complicated, they are very important as far as modern filtration methods are concerned. In fact, without them, a very popular filter, the Bakki Shower would be nothing more than an expensive waterfall.
So what is so special about these bugs? Their long scientific name helps answer that question. Facultative anaerobic can be roughly understood as meaning that they have the “facility” to live anaerobically (where there is no oxygen) provided they can get a supply of oxygen by “stealing” it from chemicals that are dissolved in the water. The expression “chemo-litho autotrophic” means that they gain energy from chemicals. The full description of their lifestyle is far more complicated than that but this simplified explanation will be sufficient to understand how these bugs can help improve pond water quality.
Nitrate has the chemical symbol NO3 which means that it is one atom of nitrogen with three atoms of oxygen stuck to it (N + O3). If we put a colony of facultative anaerobic bugs somewhere where there is no oxygen but plenty of nitrate, they will get their oxygen by taking it from that nitrate. If all three oxygen atoms are removed, all that will be left will be the single nitrogen atom. This initially will still be dissolved in the water, as was the nitrate but, since nitrogen is a gas, it will bubble away into the atmosphere at the first opportunity, in the same way that carbon dioxide is removed, by aeration.
Trickle towers versus Bakki Showers
Although Bakki Showers and trickle towers use the same bugs to reduce nitrate levels, their methods of operation are entirely different. Bakki Showers and similar filters are sometimes incorrectly referred to as trickle towers but there is a clear distinction. Bakki Showers will be described in greater detail in the next part of this series when we move on to some of the more advanced filters than the basic biological filters I have described so far. For now it’s only necessary to note that the clue to the difference between them is in the name. A Bakki Shower really needs a good flow, or shower, of water over it and a trickle tower can only work if there is a trickle of water through it. Any more than a trickle and it would no longer function as a nitrate reducing filter.
Constructing a trickle tower
Figure 3 Trickle tower design
In drawing the diagram in figure 3 to show how a trickle tower operates I have included sufficient detail so that it could be used to give sufficient constructional details to make one.
There are few details that have to be precisely followed, the design can be modified to suit availability of space and materials provided the principle of operation is observed. The important feature of a trickle tower is that pond water containing nitrate should be trickled slowly over media in an airtight tube so that no oxygen can enter from the atmosphere.
This design overcomes the many disadvantages in similar designs by making sure that no oxygen can enter either at the top or the bottom. If you plan to build it, although dimensions aren’t crucial, the way in which the water enters the tube and leaves it are important, do not alter these in any way that will allow even the slightest amount of air to enter the tube.
The tray at the bottom is a simple way to effect a water seal but it is crucial in order that air be prevented from entering.
The diameter of the main tube should be a minimum of 4” meaning that left over lengths from bottom drain pipe-work may be used although a larger diameter pipe would be better if available since this will allow space inside for more media which will allow a higher flow rate through it. The tube length isn’t too important; the minimum media bed depth is around one metre but, if a longer tube is possible, the increased depth of the media bed will allow a higher flow rate through the unit.
Holes should be drilled in the top of the main tube to allow a slotted pipe to be pushed through and made airtight with silicone mastic as shown. An end cap should be push- fitted onto the pipe to seal it. It shouldn’t be glued. A thin smear of petroleum jelly will make a waterproof seal and the cap can be held in place by a self tapping stainless steel screw as shown. In this way, the cap can be periodically removed to check that the slots in the pipe haven’t become blocked and to clear them if necessary. Water from the main filter pump is fed into this pipe by a half inch hose via a hose tail fitted into another end cap. Food grade hose should be used to prevent toxic chemicals leaching as might occur if normal garden hose was used. A valve should be included somewhere in the feed in order that the flow can be adjusted so that the water only trickles slowly over the media below.
The media bed
Typical examples of media sold under the generic name Bio Balls
Four equally spaced 6mm stainless nuts and bolts should be fitted through the body of the tower to support a media grid which in turn will support the media.
Any free draining media such as Bio Balls will be suitable. There are many products sold under that name each claiming to have a huge surface area for bacteria to colonise. As far as this design is concerned, there isn’t sufficient difference between them for me to recommend one in particular as being able to outperform all the others. Any one will perform adequately as a media in this trickle tower.
So far, my design has followed traditional lines but this one has an advantage not usually seen on other designs. Normally the bottoms of other designs are sealed and have water exiting via a pipe and either dribbling back to the pond or into a suitable filter bay for those who would prefer not to have something this large standing on the pond copings.
This concerns me since the slow flow through these units will eventually allow a build-up of silt at the bottom with no way to remove it without complete disassembly. Also the pipe exiting from the bottom will allow air to get inside and the unit will never truly be anaerobic.
This design solves both these problems by having the bottom of the unit open but standing in a suitable tray such as a plant pot tray or cut down bucket. The water that collects in the bottom of this tray before it can run away through the pipe in its side will form a water seal and prevent fresh air getting inside.
Also any build up of silt in the tray will be immediately obvious and can be removed simply by lifting the unit off and emptying the tray. The bottom of the tube should have a few notches cut in it so that it can’t form a perfect seal with the tray.
Trickle towers must only have a trickle of water through them. This is important because the incoming water will contain oxygen which will allow normal ammonia and nitrite bugs to live in the area in the media bed coloured blue and they will use all the available dissolved oxygen in the incoming water. Reducing the incoming flow to a trickle will confine their activity only to the top few centimetres of the bed. The real work of removing nitrate will be done by the facultative bugs in the area of the media coloured mauve. Since all the available oxygen will have been used by their cousins above, they will have to switch to facultative mode and obtain their oxygen supply from the nitrate in the water. Although we are cruelly starving them of oxygen in a trickle tower, they will repay our unkindness by removing nitrate for us!
Trickle filters take a long time to establish so the best way to set up this or any other trickle tower is to initially set the flow rate literally to as slow a trickle as is possible and then, after a few weeks, check the nitrate level in the outlet water. Eventually it will be lower than 5 mg/L. As the nitrate level nears zero, the flow may be increased very slightly and the level rechecked a couple of weeks later. As the filter matures, and more and more facultative bugs take residence, the flow can be increased but the level in the outlet water must never be allowed to exceed 5 mg/L. As long as this is true, the maximum amount of nitrate removal will be happening inside the tower.
Next month, more facultative bugs as the topic moves on to different types of media, but I’ll keep it simple and explain how, although they must only have a slow water trickle inside a trickle tower they are perfectly happy under the gushing flow of a Bakki Shower. How can that be? Find out in the next issue.
After this part was published there was a good response to it with many questions being asked about building a trickle tower to this design. Also one koi keeper, Simon Morris, not only went on to build one but was kind enough to send step by step pictures of the build.
Click here to read the questions and my answers and the pictures of Simon’s build.