When reputable manufacturers produce koi food, they aim to ensure that it fulfills all the dietary needs of koi and they proudly put the list of ingredients on the package or bucket, but is that nutritional information still valid on the day the fish eat it? As long as the “use by date” isn’t exceeded, there will be no threat to the fish resulting from them having eaten food that has gone off but that doesn’t mean that it won’t have lost some of the nutritional value that it had on the day it was first opened.
Vitamins are organic compounds and they will begin to degenerate when the bag or bucket is opened and they are exposed to the air. The same process begins as soon as proteins and oils in koi food are exposed. Manufacturers try to overcome this by choosing ingredients that resist degradation but there are limits to what even the most conscientious manufacturer can achieve. Whilst correct storage will minimise any nutritional losses, I would argue that supplementing a staple koi food with additional fresh food can only be an improvement to the koi diet.
The expressions “essential minerals” and “essential amino acids” are often used in relation to diet or nutrition. The word “essential”, not only means that the particular nutrient is necessary for a healthy metabolism but it also means that the body cannot manufacture it from other dietary nutrients. Therefore if any particular essential mineral or amino acid isn’t present in sufficient quantities in the diet of a fish or other animal there is no way they can obtain it by metabolic processes from any other nutrient.
To avoid a deficiency in the human diet we are advised to eat a wide range of foods in order to increase the range of vitamins, minerals and other nutrients our bodies have available to use. If that advice makes sense for me, it makes sense for my fish. For this reason I would always recommend that koi are fed primarily on a good quality koi food but supplemented by a wide range of fresh foods.
Prawns are a good source of protein as are earthworms from the garden provided there is no chance that they may have been exposed to any garden fertilizers, herbicides or pesticides. Fresh fruit of any kind is a sure source of additional vitamins.
A highly recommended fresh food for koi is lettuce. It is rich in vitamins A and C, and contains essential minerals and significant amounts of all ten amino acids that koi need. There are different views on how to feed it to koi, one way is by tearing off individual leaves, another is to throw a whole lettuce into the pond and allow koi to tear it apart themselves. The only word of caution I would add to this method is that I did hear of one large koi who, after all the leaves had been eaten, thought he would eat the remaining stump. He managed to get it into his mouth but could neither swallow it nor spit it back out. It remained stuck and had to be removed under anaesthetic. If you have a koi large enough to do the same, it might be better to avoid that risk by first removing the stump.