N: Pond Keeper’s A to Z: Netting to Nutrition

Netting (Noun)

Netting is a must to prevent koi being attacked, injured or removed by predators, be they herons or cats. Usually sold in a similar way to liners, where larger expanses are cut and sold off the roll, and smaller sizes are available pre-packed.

A dark mesh prevents it from detracting too much from the overall visual impact of a pond. It should be pulled tight to allow any leaves that fall on it to blow straight off again. To be completely effective against herons, it should be 8-12 inches above the waters surface to prevent even the most brazen of herons from spearing unsuspecting fish.

Netting (Verb)

Netting and bowling or bagging koi is a skill that only experience can teach how to carry it out in a quick, safe and stress-free way as possible. Fish should be encouraged to swim towards a hand-net, (rather than chased by a net). This is achieved by positioning a net in the direction in which a koi is swimming, capturing it head-first.

Hand-nets should be as soft as possible to avoid abrasive action on skin and mucus, with not too much of a bag from which a koi may be difficult to retrieve. Experience also shows that a darker mesh spooks koi fish less than bright white meshes and makes catching fish much easier. In addition, as large a mesh as possible should be used as it reduces the drag as you position your net strategically through the water.


A much larger net is used when harvesting koi from mud production ponds. Such nets are called seine nets and are typically the width of a pond/ lake which is encircled by the nets-men, at least one at each end of the net. The seine net will have a row of floats on the top (resembling a string of sausages), and a leaded-line on the bottom to ensure that it drags along the muddy pond bottom, taking most of the koi with it.

The seine net is pulled gently from one bank to another, and as it reaches the opposite bank, the long net (anything up to 100m long) is pulled onto the bank until approximately a 10 metre loop of net remains in the water, with the whole lead-line safely on the bank. This creates a bag in which all fish that have been netted are retained. These can be netted at will using hand-nets.

This may be carried out 2-3 times in a pond, with diminishing catches each time. Typically 80-90% of fish will be removed this way. The rest being harvested upon draining.


The term nishikigoi is rarely used in koi keeping as we prefer the ease with which its abbreviation, koi, rolls off the Anglo-Saxon tongue. (See A-Z Issue….Koi).

Nishikigoi literally means coloured carp, and relates to the whole ‘race’ of mutated and selected coloured carp, pioneered and excelled at by the Japanese.

Nishikigoi is a description for these ‘living jewels’ but it also describes a way of life for many people across the world. Sometimes described as a cult, the speed at which the Nishikigoi bug can take hold means that the Japanese have a lot to answer for (and a lot to be thanked for!).

Nitrogen Cycle

It is with all sincerity that we should thank the scientific fraternity for the close association between a number of words that all begin with the prefix ‘nitr’. The link between them is the element nitrogen, with the imaginative scientists using ‘nitr’ to describe the majority of words that link the processes and forms that this key element has in the life and health of a pond. I.e The odds are, if a word starts with ‘nitr’ it has got something to do with the nitrogen cycle.

Nitrosomonas Nitrobacter

Koi keepers must be competent chemists to be able to understand and respond to a number of key parameters in the water. Consequently, we should try to understand as much about nitrite, nitrate, Nitrosomonas, and Nitrobacter, (all part of the nitrogen cycle!) and how they affect koi.


What is nitrogen,and why is it so important that it lends its name to so many other terms in koi keeping?

Proteins are made up of amino acids, which contain nitrogen. Nitrogen enters the pond every time koi are fed as it is found in all proteins. Unfortunately, nitrogen has very limited uses in the body, (probably why 78% of our atmosphere is nitrogen!) and once it becomes removed from amino acids, must be excreted – in the form of ammonia NH4+. Once koi have digested the protein in their diet, they will utilise it in one of two ways, depending on a number of factors.

Firstly, after digesting and assimilating protein from the diet they ideally redeposit it in new tissue growth. Very simply, this is how koi grow and is why koi should be fed higher protein diets in the summer. A less desirable destination for protein is when koi burn them off as a source of energy. In doing this, they will utilise the amino acids as fuel, but excrete the nitrogen component as it has no energy value. This is undesirable for 2 reasons. Firstly, costly protein is being used by koi as a source of energy (when cheaper carbohydrates or oils could be used) and secondly, the nitrogen that is discarded is excreted as toxic ammonia.

Ammonia is toxic if allowed to accumulate, and so is excreted to reduce toxicity making the pond water potentially toxic. Fortunately, one mans muck is another mans brass, and a host of bacteria, including Nitrosomonas bacteria are on hand to process the toxic ammonia into less toxic (but more persistent) Nitrite. This occurs quite quickly in a pond (and is the reason why a nitrite reading is far more common than an ammonia reading. Nitrite is broken down more slowly by Nitrobacter bacteria, and tends to be more persistent than ammonia. However, once it has been broken down, and the Nitrobacter colony matures to cope with the quantity of nitrite produced by Nitrosomonas, a pond bio-filter has passed through its most ‘risky’ stage and appears to be able to handle waste more effectively.

Nitrobacter oxidise (add oxygen to) the nitrite further, forming less toxic nitrates (still with nitr prefix – denoting a nitrogen link). These nitrates are available and taken up by plants, to produce plant proteins. (Which are then consumed by fish, either incorporated as fish flesh or burnt as energy, excreting nitrogen as toxic ammonia, and the cycle continues).

If it is not practical for plants to be incorporated in a pond or even as a vegetable filter, then nitrates can be reduced, either by diluting them through a partial water change, or through the use of a nitrate filter.


A nitrate filter, is an area of slight anaerobic conditions, where water flow is reduced, and media is completely saturated with water of a low DO. (Really the opposite of what we have come to regard as effective filtration). In such conditions, anaerobic bacteria (bacteria that prefer not to utilise oxygen), will predominate, and acquire oxygen from that which is found in nitrate molecule (NO3). This reduces it eventually to N2 gas which is bubbled off into the atmosphere (to join the other 78%). Unlike the aerobic Nitrosomonas and Nitrobacter bacteria, these anaerobic denitrifying bacteria require food, and can be fed with soluble organic food, such as ethanol (alcohol).







Converts NH3/NH4+ into NO2

Fast flowing aerated water



Converts NO2 into NO3

Fast flowing aerated water

Pseudomonas denitrificans


Converts NO3 into N2

Slow flowing de-oxygenised water



After water quality, koi nutrition has the greatest influence on the health and appearance of koi. Nutrition can also have a significant influence on water quality.

By providing the correct nutrition, koi will improve in their growth, colour, resistance to disease and breeding performance. Get it wrong, and a koi’s health, growth and colour will be compromised.

Arguably, when describing a koi’s diet, the term ‘food’ is concerned with its physical properties; how it looks, whether it floats and how it behaves in the water. When discussing a koi’s nutrition, the diet’s nutrient profile, specification and performance are being described.

Nutrition describes aspects such as levels of protein, carbohydrate, oil, vitamins and minerals. As a koi’s nutritional requirements develop with age and season, so should the specification of the diet. Consequently, koi are not fed the same diet throughout the year, but are offered wheatgerm-based low protein diets in the cooler months and higher protein ‘growth’ diets when it is warmer.

Unlike the feeding of any other pets, the nutrition offered to fish is also likely to affect their environment. Better water quality can be maintained by fully appreciating that when koi are fed, so too is the whole pond system. The more food that is offered, the more biofilter has to detoxify. That is, the more protein that enters the pond, the greater the risk of a water quality problem.

Kill blanketweed and string algae.