Most of us feel that we have a fair understanding of how to feed ourselves in a healthy manner (though we may not always follow this knowledge!). This comes both intuitively and from the nutrition education we have received all our lives. We can extend this understanding to also effectively feed our common, mammalian pets, such as dogs and cats. We cannot, however, feed our fish in the same way without endangering their health. The first step in deepening our understanding of koi and goldfish nutrition is to understand exactly how and why fish are so different.
Constraints of the Aquatic Environment – Temperature & Oxygen Levels
The aquatic environment differs greatly from the terrestrial environment and these differences directly affect how fish are nourished. Let’s take a look at these differences in light of their affect on nutrition. First consider temperature: even a small pond in a harsh environment will have a daily fluctuation of less than 10 °F and most ponds will change only a few degrees from day to night far less than the air. Similarly, the aquatic environment is buffered from seasonal fluctuations in temperature. Moreover, the coldest that the aquatic environment can physically become is 32 °F (freezing). While fish have less temperature change with which to cope than terrestrial animals, they often suffer from a lack of oxygen availability. The concentration of oxygen in air is always 260 mg/L. On the other hand, the concentration of available (dissolved) oxygen in water is, at most, about 14 mg/L and may be much less, even 0, depending on a variety of circumstances including temperature and bacterial activity.
The Ammonia Angle
Besides being cold-blooded, fish are different from terrestrial animals in another way. When any animal metabolizes protein, it creates waste ammonia, a toxic compound. In order to detoxify ammonia, mammals build it into water-soluble urea, a safer compound to circulate in the body until excreted. Fish, on the other hand, excrete ammonia directly into the water. In crowded pond conditions, ammonia or its degradation product nitrite can build up and poison the fish. Ammonia production is directly linked to feeding.
The Aquatic Food Requirements are Different
Besides the temperature stability and oxygen concentration, the aquatic environment differs from the terrestrial one in that most plant production in the water is done by microscopic phytoplankton too small to be eaten even by the tiniest tosai. That means the herbivores in the aquatic environment are small zooplankton, which are eaten by small fish. The upshot of this is that fish are nearly all carnivores, which includes koi and goldfish, contrary to what many lay people, and even many biologists, think. That doesn’t mean koi and goldfish are predatory like a trout or bass: they are in fact grazers, but the material they graze on is very high in protein, about 50% based on studies of the diet of wild carp. Also note that the scientifically determined, minimum protein requirement of koi (35%) diets is higher than cats (30%) and even dogs (20%). In the natural aquatic diet, the high protein comes at the expense of carbohydrate, which means koi, and to a lesser extent goldfish, do not tolerate high levels of carbohydrate in the diet, again this contrary to accepted belief.
Another dietary distinction of aquatic environment, at least in temperate areas, is the lack of saturated fats. Saturated fats, as opposed to oils, are solid at room temperature. The reason koi and goldfish can’t digest them is simple: they would be a hard lump in their gut at most pond temperatures.
Total Protein Requirements as % of Diet (dry)
Channel Catfish 32
Common Carp 35
Rainbow Trout 38
Pacific Salmon 38
Saturated Fatty Acids
Brook Trout 26
Unsaturated Fatty Acids
Brook Trout 72
Take Home Lessons
So how does our knowledge of fish and the aquatic environment help us understand how to feed our koi and goldfish? Essentially, cold-blooded animals need less food than similarly sized warm-blooded ones for maintenance of body weight. The fact that fish need less food to live is part of the reason overfeeding is such a problem with beginning fish keepers. The other reason is that when one overfeeds the dog, the uneaten food sits in the bowl with little consequence. Not so in the water: the ignored food soon disperses and begins contributing to the organic load of the pond which results in the decline of dissolved oxygen and an increase in ammonia through bacterial decomposition.
Feed less, much less, than you would a comparably sized warm-blooded animal. In adult fish, this is 1 to 3 percent of body weight a day depending on temperature (more in warm temperatures). Do not feed to the point that uneaten food remains in the water. A rule of thumb is all food should be consumed in about 5 minutes, several times a day.
Feed high protein (about 40% for koi, 30% for goldfish), low carbohydrate food (less than 30% for koi, less than 40% for goldfish). Lipids should be oils, not warm-blooded animal fats, preferably from marine sources.
Since koi and goldfish are grazers, they do best with many small meals. Fish in the wild eat almost 24 hours a day. While this is impossible in a pond, it is good to break the total daily ration into as many small feedings as practical, even employing an automated feeder.
While high protein feeds are recommended, ammonia production is linked to protein in the diet and this makes a biofiltration (nitrification) system for ponds essential. Ammonia peaks in the water about 90 minutes after a meal. The frequent feedings indicated above help avoid excessive ammonia spikes.
A final tip: fish, unlike warm-blooded animals, can be fasted with no ill effect for several weeks. Fasting also minimizes water quality emergencies.
Remember with vacations coming up give your customers feeding tips for the person they have taking care of their pond. First, koi can go several days without being fed. If your customer is going to be gone for more then a week or two, suggest to them to pre-measure the food that is to be fed daily by meal, so there is no guessing by the vacation helper. This usually is a smaller quantity to again reduce the possibility of over feeding. They can either put each feeding in a zip lock bag or have a cup with a clearly marked line as to how much to put in the pond each time.