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Point of view       (Points of View part 2)


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

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A series of six short articles commissioned on a range of subjects

1. Overwintering koi
How can we over winter koi without the bugs in aeromonas alley eating them, and what is aeromonas alley anyway?  Aeromonas alley describes the difference between the ability of pathogenic (harmful) bacteria, such as aeromonas, to infect a fish and the ability of the fish’s immune system to defend against such infections. There are several species of aeromonas bacteria. Four are commonly found in natural bodies of water, and also in koi ponds. They are opportunistic bacteria that infect minor wounds, such as parasite bites, causing ulcers, fin rot or tail rot.  If left untreated, skin ulcers will grow until the muscle tissue is affected and this allows the infection to spread through the blood stream causing organs such as the kidneys to fail, resulting in the condition known as dropsy, or pine cone disease. Koi rarely survive this.

The activity of bacteria and the efficiency of a fish’s immune system are both controlled by water temperature.  Luther Chien, a retired Dupont Corporate Science Fellow, studied this effect and concluded that below 4.4°C both aeromonas and the koi immune system are inactive. Koi in a pond at that temperature have little defence against bacterial infections but are safe because aeromonas and similar pathogenic bacteria are dormant. As the water temperature begins to rise, aeromonas slowly become more active and take any opportunity to feed and multiply. At these low temperatures, koi immune systems aren’t fully effective and so, until temperatures rise significantly, aeromonas can infect koi unhindered. Chien describes the range 4.4°C to 12.8°C as “aeromonas alley”.  Others believe this range is a bit wide but most agree that the range 6°C to 8 or 9°C very much favours aeromonas and puts koi at a disadvantage.

Understanding aeromonas alley allows us to form a strategy for over wintering koi. One way is to heat your pond so that it never falls below about 10°C.  Not everyone can afford this but I believe that some degree of heat is necessary, so an alternative is to allow the temperature to fall naturally during winter but to provide just enough heat to prevent it falling below 4°C. As soon as water temperatures begin to rise in spring, slowly turn up the thermostat to raise the temperature until it is above 10°C, ensuring that the fish spend as little time as possible in the risky area of aeromonas alley and minimising the risk of an aeromonas infection.


2. Water quality
According to OATA, poor water causes around 90% of fish deaths, so it is important to check the parameters but which are most important? In my opinion, there are six parameters that should be checked regularly.  Ammonia, nitrite, nitrate pH, KH and oxygen.  Keeping these within acceptable limits is the key to good water.

Dealing with oxygen first, I check it daily but only very rarely test with a meter or kit. Every time I look at the pond, I see air pouring into it from a large flat ceramic air diffuser.  So much air is being fed into the aerated K1 that it is almost jumping out of the water, so whenever I have tested the oxygen level with my oxygen meter, it is always at “saturation point”.  Under these conditions it isn’t necessary to test for oxygen with a meter or a kit, I can see that the fish are getting plenty of oxygen every time I walk past. If, and only if, your pond is similarly aerated, there is no need to test what is obvious every time you look at it. If not, a test kit should be used to confirm that the oxygen level never falls below 6.0 mg/L.

Of the other five parameters, ammonia and nitrite are obviously important to test. Both are pollutants that will seriously harm or even kill fish if they are not kept below the recommended limits; 0.02mg/L for free ammonia and 0.2 mg/L for nitrite.

Nitrate is usually regarded as a nuisance that will encourage algae but I don’t think it has had the prominence as a pollutant that it deserves.  It is true that it has to rise to very high levels before it will kill adult fish, but research is now showing that even comparatively low levels will delay or even prevent fish eggs from hatching and there is good anecdotal evidence from koi breeders that the lower the nitrate level, the better the growth and colour development.

pH and KH are two halves of the same parameter.  Koi will adjust to any pH in the range 7.0 to 8.5 but the actual value shouldn’t vary by more than 0.2 per day. The key to keeping a stable pH is to have a sufficiently high KH.  The way KH regulates pH is too complicated to explain in this short article but KH is a measure of carbonate dissolved in the water.  Carbonate is fascinating, (for some of us); according to the pH, it will change into bicarbonate, carbonic acid, or carbon dioxide and back again.  In changing back and forth, it acts as a brake on pH variations. Biological filters also need dissolved carbonate to function.  If the filter should use it all, the pH will crash which is a potentially life threatening situation. Sufficient carbonates may well be introduced with water changes but this needs checking. An ideal KH is 100 mg/L.


3. Koi growth rates
It will come as no surprise that I would put water quality as an important factor in affecting Koi growth, but assuming that water parameters are at optimum values, and that good quality food is being fed to them, what else could improve or retard the ability of koi to grow?

Whilst many animals have a growth pattern that is limited to a maximum size, fish continue to grow throughout their lives virtually without limit. In early years, fish growth rates can be very rapid but, as the fish grows larger, the rate slows until it hardly appears to be growing at all because, as they increase in bulk, their increased body tissue means they will have an increased requirement for oxygen.  This has to be provided by the gills which do not increase in surface area to match the rate at which fish “pile on the pounds”. If a fish doubles its length, its body mass increases by a factor of eight, meaning that, if a 1 kg fish doubles its length, it will then weigh 8 kg, but the gills are not eight times larger than when it was only 1 kg.

Larger fish are not “suffocating” due to their extra bulk, but as the fish continues to grow, the reducing supply of oxygen per kilogram of body mass acts as a limiting factor on the rate of growth.  Another important factor is the fish’s ability to metabolise its food.  This process requires yet more oxygen for it to get the full benefit of protein and other nutrients in the food.  If sufficient oxygen isn’t available, food will be excreted only partially digested.  Keeping the dissolved oxygen level as high as possible in a koi pond will allow good growth rates to be achieved.

Koi metabolism approximately doubles for an increase of 10°C.  Increasing the temperature will therefore increase their appetite. However, increasing the temperature can lead to reduced growth rate if food is not offered to match this increase in appetite because food is firstly used to run the fish’s metabolism, then secondly to maintain or repair existing body tissue. Only nutrients that have been metabolised, and which are left over after those two processes, are available to be used for growth.  At higher temperatures, feeding little and often throughout the day will ensure that all nutrients in the food are fully metabolised and available for growth.


4. Small world
As far as koi-keepers are concerned, there are two main types of bacteria. The first type are called heterotrophic bacteria and they obtain their energy to grow and reproduce by metabolising (the bacterial equivalent of eating) organic molecules, which are molecules that were once part of something that either was, or is, alive.  Some heterotrophs confine themselves to metabolising fish faeces or dead plants and are harmless, but others also regard the living tissue of fish as part of their “diet”.  These particular heterotrophs are by no means harmless and include ulcer causing bacteria such as aeromonas and pseudomonas.

A healthy koi’s immune system is capable of warding off low levels of heterotrophic bacteria but high levels will overcome its defences.  Heterotrophs are so numerous in the environment that they cannot be kept out of a koi pond entirely, and if conditions in the pond suit them, they can multiply to epidemic proportions very quickly, so the best way to keep levels low is to deprive them of their “food source” by keeping the pond and filters clean.

The second type of bacteria that concerns koi keepers are autotrophic bacteria.  These obtain their energy from purely chemical sources. Ammonia is a chemical that is excreted by fish as a waste product and the two most useful autotrophs found in a koi pond are the bugs that convert this ammonia to nitrite (nitrosomonas) and those that convert the resultant nitrite to nitrate (nitrobacter).  They are not fussy about which media they will grow on, they will grow on any wet surface in the pond but they don’t like light so they will do best on a media that is porous or in a bio-filter that is kept dark.

To convert one ammonia molecule to nitrate, they need four oxygen molecules, so the bio-filter must be well aerated and since they can only grow and multiply by doing complicated chemistry, the rate at which they multiply is very much slower than their heterotrophic cousins. The spike in ammonia or nitrite levels in an unheated pond in spring is simply the result of the koi’s appetite increasing rapidly and their resultant increase in ammonia output. Nitrifying bacteria will ultimately multiply to take advantage of the increase in their “food source” but they can only do this slowly, so koi-keepers should resist the urge to feed their fish heavily as soon as their appetites return after winter.  


5. Gravity or pumped filters
I personally prefer gravity filtration to pump fed systems and I think the majority of koi-keepers feel the same way, so to answer the question; “pump fed or gravity?” it would be all too easy to answer by extolling the virtues of gravity filtration. Instead, I will put the case for pump fed filtration since, under some circumstances, it is the better option.

In-ground ponds: It is difficult to design an in-ground pond that looks like a natural lake without using a pump fed filter system. Gravity filtration requires the water level in the filters to be the same as the water level in the pond, so for gravity filtration with an in-ground pond, the top edges of the filters have to be approximately at ground level. This would involve a lot of bending and kneeling during maintenance or to have the filter system in a pit that would be accessed by steps or a ladder.  For in-ground ponds, a pump fed system is easier to install and maintain. 

New koi-keepers with limited budgets:  The cost of building a koi pond with state-of-the art filtration may be daunting for many intending koi-keepers. Putting a liner into a four foot deep hole and feeding the filter from a submersible pump may seem much more affordable. 

These are just two situations where pump fed is best but there are others. If the pump fed option is chosen, there are some special considerations.  Pumps should not stand directly on the pond floor.  Firstly, pumps vibrate and, over long periods of running, this vibration can slowly wear through a liner. Secondly, if a leak occurred, or if anything should cause the filter to over flow, the pond would be completely drained. Thirdly, for deep ponds, the extra water pressure on the pump shaft’s water seal can cause it to leak and allow water to enter the motor.  It is better to stand the pump on a raised platform or suspend it away from the bottom. When suspending a pump, it must not hang by its electrical lead because it enters through a water-proof gland which will leak if the cable is pulled.  Also submersible pumps need to be regularly removed for cleaning so they should be positioned where they are easy to reach. If these factors are taken into account, there is no reason why pump fed should be regarded as an inferior system.


6. Energy consumption
Whether we do it in order to minimise our carbon footprint for the sake of our grandchildren’s planet or the more selfish reason – for the sake of our bank balance, we should all be making the most efficient use of the energy needed to run our koi ponds.

One obvious way to save energy is to use a lower power pump, but choosing the best pump for a filtration system is not as simple as picking the pump that has the lowest power rating. There is nothing that one pump manufacturer knows about pump efficiency that all the others don’t also know and so, due to commercial pressures not to be the least efficient pump on the market, all modern pumps are made to similar efficiencies.  Designing new pumps involves making compromises about power consumption and flow rates against back-pressure. A very low power pump that is able to pump the same number of litres per hour as a higher power pump with the same flow rating will certainly give equal performance in simple filter systems.

However, the flow rate of very low power pumps will fall far below that of higher power pumps when they have to pump against a significant back pressure or lift water to the top of a waterfall or shower. In these situations, the turnover rate will fall so that, where a two hour turnover might have been expected, the system may, in fact, only produce a three hour turnover. Calculating the losses in pump performance may seem daunting but there are dealers who offer to do this for you and it may be that the second lowest power pump on the shelf is the better option for your particular installation.

A very efficient way to move water through a filter system is by means of an airlift.  This is where air is fed into the bottom of a vertical pipe and, as it rises, it causes an upward flow of water. This is most suitable for new installations because the layout of the pipe-work must be correct or it won’t work.  Airlifts also can’t pump water significantly above the pond water level but, with the correct design, an airlift can feed an amazing amount of water into a filter system. This water will also already be highly aerated making it unnecessary to provide additional aeration for the bio-filter, saving just a little more energy.

Until recently, two hour pond turnover rates were recommended. With the ammonia produced by typical koi foods and average rates at which bio-filters can remove ammonia, a two hour turnover is sensible.  Possibly even a 1 hour rate is desirable, but on some forums, there seems to be a competition over who has the fastest turnover rate, and I recently saw a post where someone stated they had achieved a 20 minute turnover.  I think the pursuit of ever quicker turnover rates is unnecessary and energy could be saved by pausing and questioning whether rates faster than one hour actually serve any purpose.


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