By Rick Kleyn (MSc Agric) – Animal Nutritionist

I am a new member of the South African Koi Keepers Society and new to keeping Koi. I have just bought my first fish and put them into my new pond. What then to feed these beautiful creatures? There is a dazzling array of food on the shelves in our pet and Koi shops, both locally made and imported. Prices range from less than R 30.00 per kg to well over R 80.00 per kg so they can’t all be the same – or can they?

I may be new to Koi keeping but I am an agriculturist with some experience in nutrition. My instinct is to look at the labeling on the packaging and then to make my selection based on the information that it contains. This has proved to be far more difficult than I thought it would be for two reasons. Firstly many of the labels are inadequate and would fall short of the minimum label requirements for farm animals or for that matter most processed human food. Secondly, even when the labeling is of the highest quality, there are some issues that are simply not addressed.

Before I go into detail about the labels, let us remind ourselves of just what it is that we require in a diet.

  • Although the fish will gain some nutrients from the pond ecosystem, it is probably safe to assume that the fish are pretty much entirely dependent for nutrients on the food that we supply them with.
  • The diet needs to be balanced. In broad terms this means that it should contain enough protein for normal growth together with adequate energy for the fish to utilise it correctly. It should also contain adequate levels of vitamins and minerals that are required by the body for normal metabolic function. Although many of these are only included in small amounts they are the essential “keys” for normal function and any shortfalls in this area will lead to the ultimate death of an animal.
  • Fish have different nutritional needs at different stages of life.
  • Ideally the food should float.
  • Colour is no indication of feed quality or indeed of what colour the fish will attain.
  • The food should be fresh.

The guaranteed analysis is a statement that should be printed on each bag of food sold. It should provide concentrations of specific nutrients. Fish owners should be able to use the guaranteed analysis to select food correctly to meet the nutrient requirement of their fish. The problem is that it is possible to print any information that you like on the label, as in this country there is no regulatory control. Without subjecting a sample for chemical analysis it is impossible for the consumer to verify any label claims. It helps therefore to buy from reputable manufacturers and/or suppliers.

In the rest of this article I will look at each of the important aspects of a label in turn.


In general, people place unwarranted importance on the protein content of a diet. Food is often purchased solely on the crude protein percentage, with little or no concern for the other important nutrients. It is true that fish have a higher protein requirement relative to energy than land based animals. This is because they are cold blooded and don’t need as much energy to maintain body temperature and also because they require less energy to move about in their environment.

Most bags of Koi food that I have seen do mention the protein content of the diet on the label. In some cases this is the only thing mentioned. In nutritional terms protein means little, as it is the building blocks of protein, the amino acids, that we are interested in. The protein quality of a diet is determined by the relative level of each amino acid that it contains (called the amino acid profile). Thus a diet containing 32 % protein with a low level of an amino acid such as lysine may well be inferior to a diet containing 28% protein with a reasonable lysine level. If the diet is manufactured using fishmeal as the primary protein source, then the amino acid profile is seldom an issue. If however, the product contains a high percentage of plant protein sources this is something that needs to be considered.

Contrary to what some people believe, fish do not prefer to use protein as an energy source, but rather are able to break down surplus protein into energy yielding carbohydrates and waste ammonia. Unlike other animals the ammonia does not need to be transformed in to urea (mammals) or uric acid (birds) at a relatively high-energy cost. Protein catabolism (breakdown) will only occur if there is insufficient energy in the diet, or if the protein is of a poor quality (poor amino acid profile) and can’t be used effectively for tissue growth and/or maintenance by the fish. Not only is using protein as an energy source expensive but it also leads to the pond being polluted with ammonia.

It is a well-established scientific fact that small fish require relatively more protein, in reality these are amino acids, than do larger fish. This is for two reasons. Small fish have a relatively high growth rate, when measured in terms of percentage weight gain. Larger fish, simply because of their size, have a higher energy requirement for maintenance. Feeding high protein diets to mature fish therefore makes little sense, and in most cases we probably overfeed protein.


As already mentioned, fish require an adequate amount of energy in their diets if they are to utilise the amino acids supplied in the diet. Energy in most diets is supplied in the form of carbohydrate, for example the starch in maize. Fats and oils are energy dense and providing nearly 2.5 times as much energy, gram per gram, than carbohydrates or protein. This means that the higher the minimum percentage of fat in a food, the higher the calories provided per gram of feed. The higher the energy content, the fewer grams of food is needed to meet energy requirements. From a more practical perspective, fat is by far the most expensive component of the diet, which in effect means that diets low in fat are far cheaper to manufacture. Foods that are high in fat generally contain higher levels of fishmeal. Remember, that fishmeal contains high levels of fat, so the fat content may well be an indicator of the protein quality as well. In broad terms therefore it is the fat content of the diet that is likely to indicate it’s ultimate quality rather than protein.


Fish do not have the enzymes to utilise fibre, so any fibre included in the food simply passes through the animal. High fibre levels in a diet indicate that the food contains high levels of “filler” material such as wheat or rice bran, which in turn means that the energy level of the diet is low. From a manufacturing perspective, it is easy to extrude diets that are high in fibre. They also tend to float better.

Vitamins and Minerals

The vitamin and mineral content of a food are critical, as these are the key nutrients that govern the various metabolic pathways in the fish. Adequate levels are required in the food and this should be reflected on the packaging. There are some practical problems with vitamins in particular. They are sensitive to both temperature and light. As most Koi food is cooked via an extrusion process, if the correct levels and type of vitamins are not used when the feed is manufactured, the actual levels in the product you purchase may be inadequate. In addition, vitamins degrade with time. Animal feed vitamins are assumed to have “expired” within six months of the feed being manufactured. Using this as a rule of thumb it is probably safe to assume that the same would be true for Koi food. This is something that is exacerbated by exposure to bright light and high temperatures. For this reason, always store your food in a cool dark place.

As we are all aware, fish like all animals require both Calcium and Phosphorus for normal skeletal development. What is less well know is that in order for the bone calcification process to occur normally, the Calcium and Phosphorus must be in the correct ration to each other. This means that there should always be 1.1 to 2 times as much Calcium in the diet as Phosphorus. If this ratio is inverted, serious skeletal problems (twisted spines for example) will result.

Feed colour and colour enhancers

Most fish foods contain two classes of colouants. Firstly, they should contain ingredients that will enhance the colour of the fish themselves. These include such things as spirulena, shrimp meal and possibly even synthetic carophylls. Secondly, they may or may not contain artificial food colorants to give the finished product a certain colour. The food colorants used to colour the diet are of little consequence nutritionally but they may well colour the water red or green. It is the colour enhancer used that should bear more close scrutiny. The cheapest way to “colour” up fish is to use synthetic canthaxanthin. The EU have just passed Directive 70/524, which limits its use in fish food (and poultry food for that matter) as it has been shown to lead to blindness in the target species.

Physical Characteristics

There are a number of physical characteristics that the buyer of fish food should look out for. The pellets should be uniform and the amount of “fines” in the bag should be minimal. You should not be able to detect any individual ingredients in a pellet, for example chunks of maize, as this means that the product was not processed properly. A smooth finish on a pellet would indicate that it has been fat coated after manufacture. This will enhance palatability and also indicates that the energy level of the feed is probably reasonable.


Feed freshness is important for a number of reasons. When any food ingredient is milled it immediately becomes more susceptible to mould contamination. Also, the fats in the diet are inclined to go rancid over time, which can cause the food to become unpalatable. Lastly, as already mentioned the Vitamins degrade with time. Try to buy food that has a date of manufacture label on it, and avoid feed that is more than 6 months old. Keep your feed fresh by storing it in air tight containers (oxygen is the enemy), preferably in a fridge.

Some Analytical Results

I have taken the time to draw some samples of food from the shelves of local shops. These were then submitted to the University of Natal for analysis. These results are reflected below, together with the published analysis of the Pond Nuggets supplied by Mazuri Foods which I have used for reference purposes. As can be seen, the protein of some of the imported product is low and they have an inverted Ca:P ratio. The local products on the other hand are adequate in terms of their protein content.


Making the final decision

All Koi owners are feeding valuable and much loved fish. The only way in which you can make the correct decision as to which food to buy is to ensure that the information that you require about the diet is printed on the packaging. It helps to buy from a reputable manufacturer and/or supplier so that you can have some confidence that the label information is correct. You need to know how much protein or preferably amino acid is contained in the diet. You also need to know what the fat or energy content of the diet is and it is helpful to have and idea of what the fibre content is as well. It is useful to know what colorants are used in the food’s manufacture so that you avoid the potentially harmful levels of synthetic canthaxanthins. Freshness and physical form are important so you need to examine the feed yourself. It is useful to have a date of manufacture (not date of repackaging) to ensure that the feed is fresh.

Labels that state simply “High Quality – Fully Imported” with absolutely no additional information on them are meaningless. Being ”imported” is no guarantee of quality. Local product that is properly and responsibly manufactured and marketed will always compete well in terms of quality and price. Remember that the South African animal feed industry is world class. Why would the makers of local fish food be any different?

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