By Stephen M. Meyer
Carp and goldfish evolution has seen to it that the proper functioning of their digestive systems requires dietary variety over the long run. Monotonous diets are fine in aquaculture where fish are routinely culled out for market early in life, but ornamental pondkeeping aims for natural life spans, which places more subtle demands on fish-rearing techniques.
In this respect, no single food type, no matter how nutritious, represents an appropriate or healthy long-term diet for koi or goldfish. I doubt that any of the manufacturers of premium koi and goldfish food would claim that their products should be the exclusive diet of your fish. Therefore, you should make every effort to offer your koi and goldfish a varied diet (which does not mean different brands of pellets but rather different types of foods: vegetables, insects, etc.).
It is useful to think in terms of a base diet and a supplemental diet for your fish. The base diet provides the essential proteins, fats and most vitamins and minerals. The supplementary diet provides additional vitamins and minerals, but also other proteins and fats and, most importantly, variety.
The Base Diet
I believe that the most reliable, healthy and convenient base diet is a quality commercial pellet food. There are many brands from which to choose, with the protein content varying greatly depending on the manufacturer. Some pellet foods offer about 25 percent protein, whereas others are up around 36 percent. Read the labels! In any case, the primary source of protein in the pellets should be fish or soybean meal. As noted earlier, protein from animal sources or corn and other grains is less easily assimilated by fish. Fat (from fish oil) content should be at least 5 percent.
Most commercial fish food manufacturers now list vitamins added to their foods, so you can check the nutritional coverage of different products. Unfortunately, they omit information on the quantity of vitamins available per feeding, but it may be possible to solicit this information from individual manufacturers.
A question that often arises is whether it is necessary to buy "premium" koi and goldfish foods to raise healthy fish, or will any basic pellet food suffice? The answer is: it depends. In general, premium foods are not absolutely necessary. This does not mean that the foods do not perform as advertised. It just means that they are not "necessary." A basic pellet diet, supplemented as described below, is more than adequate, assuming the package is clearly labeled as to nutritional contents.
For example, a good-quality catfish fingerling food — in which the primary protein ingredients are soybean and fish meal — is similar in nutrient value to premium koi foods, yet is quite inexpensive. Trout foods, however, are not appropriate as a long-term diet for koi or goldfish.
"Color-enhancing" foods are quite popular today. Most breeders, however, will tell you that genetics and water quality have far greater effects on color than do food additives — assuming a basic healthy diet to begin with. Moreover, many of the color enhancers can be obtained in the supplemental diet listed below and at lesser expense.
Some folks make their own koi food from a variety of food stuffs and table scraps. Recipes can frequently be found in KOI USA and The Goldfish Report. If you have a small collection of pond fish, this may be a fun and interesting area to explore.
In general, however, I do not recommend that the average pondkeeper go this route for the base diet. First, getting the right mix of proteins, fats and vitamins is a bit tricky. Many of the recipes I have seen are not healthy for carp. They use the wrong kinds of protein sources, fail to include the right fats, are overly rich in carbohydrates and contain too much bulk.
Second, homemade foods have a tendency to go bad faster than commercial foods because they lack preservatives. Rancid fish food can kill fish faster than most diseases. Unfortunately, rancid fish food looks and smells just like healthy fish food.
Third, it is not clear that proper homemade foods save substantial amounts of money. Most of the money-saving recipes I have seen use food scraps from whatever is left over in the kitchen. While it is true that carp and goldfish are scavengers, they are not substitutes for garbage disposals!
No matter which approach to a base diet you take, it is important to presoak the base food in pond water for 30 seconds to one minute prior to feeding. Fish digestive tracts have evolved for moving foods with high moisture content (80 percent or more). This is especially important when the frequency of feeding is low or the fish are very hungry, because they will quickly consume dry pellets before there is a chance for pond water to be absorbed. Presoaking will lower the risk of the dry foods expanding in the digestive tract of your fish and causing potentially life-threatening blockages or internal bacterial blockages.
In the same vein, if you feed commercial pellets, I strongly suggest that you stick with small pellets. The nutritional value is the same, but small pellets are far easier to ingest and digest. There is no reason why big fish must eat big pellets, and the danger of intestinal impaction is greater.
The Supplementary Diet
The supplementary diet provides both variety plus a redundancy in nutrients beyond the base diet for an extra margin of safety. It need not be expensive. For instance, a good supplementary diet should include ordinary pond algae — especially the long stringy kind. Other natural pond plants, such as duckweed, can be collected or grown. Vegetable scraps — peas, carrots, spinach, leaf lettuce and so on — are appropriate for supplementing the base diet. As an infrequent protein supplement, you can find frozen shrimp at oriental food stores at very modest prices. Some pondkeepers add scraps from food fish bought and prepared for their own consumption (again, do not use meat scraps from beef, chicken, etc.).
Initially, it may take some time to accustom your fish to these "foreign" foods, but there are a few tricks you can try to ease their adaptation. First, always soften vegetables by par boiling (about two minutes). Never dump frozen vegetables into the pond! Second, you might initially mix the cooked vegetables in a container with particles and dust from the base pellet diet. This will impart a familiar smell and taste. Once the fish get used to the flavor and texture of the vegetables, this step will become unnecessary.
The use of homemade foods as part of a supplementary diet can work well. In this instance, considerably less care need be taken on getting the proportions right. Cod liver oil and ordinary vitamins can be added to the mix. Color-enhancing vegetables (those containing carotene — carrots, peas, etc.) can be ground in.
The burden of making the food is also significantly less when prepared as a supplementary ration. Only small quantities need be made at one time, which lessens the likelihood of it going bad.