Feeding Koi and Goldfish
The fundamentals of feeding koi and goldfish.
Stephen M. Meyer
Koi (carp) and goldfish are non-aggressive omnivores — they will eat almost anything that won't eat them first: plants, insects, snails, worms, algae and so on. Because they receive their nutritional requirements from such a wide variety of potential food sources, it is not surprising that carp and goldfish living in the wild rarely suffer from nutrition-related health problems. Their natural aquatic environment hosts and immense quantity and variety of food sources to meet all dietary requirements for proper growth, development maintenance and reproduction.
It is in this respect that the typical ornamental pond fails to adequately replicate nature. If nothing else, the fish load in these ponds (the mass or weight of the fish in relation to the volume of water) is characteristically tens to thousands of times greater than those found in the wild. Moreover, the quantity and diversity of aquatic fauna and flora in these ponds — particularly in koi ponds — is extremely limited and never in balance with the fish population except in the very rare instances where the pond is devoted primarily to plants, not fish. Thus, hobbyists cannot expect fish in the typical ornamental pond to completely satisfy their dietary needs with pond food sources alone.
This is where the art and science of pondkeeping come into play. The pondkeeper stands in for mother nature by providing the basic diet for his or her animals through daily feedings. It is essential that the daily ration of food given to the fish meets all nutritional requirements, both in quantity and ingredients. At the same time, proper pond fish nutrition and diet also depend on the feeding procedures themselves. As is explained below, correct feeding procedures are especially important in the care of koi and goldfish.
Although it would be easy to list what I consider to be the best foods to offer your koi and goldfish, and then specify how they should be fed, such advice would be of little lasting value without providing the background on which it is based. There are simply too many options and alternatives available today, with more to come in the future.
Moreover, personal finances are also relevant. The costs for food can mount up over time, and you may be spending more than necessary to provide the correct diet. Therefore, this article begins with a look at the basic nutritional needs of koi and goldfish. We will also examine some of the physiological characteristics of ingestion and digestion that are peculiar to koi and goldfish, and therefore affect the choice of food types and feeding procedures. With this information in hand, I will then offer some suggestions for feeding.
Nutrition and Diet
The basic nutritional requirements of koi and goldfish are not terribly different from those of other fish or animals. Proteins are needed for growth, development and maintenance. Fats (especially the water-insoluble lipids) represent the major energy source in the diet. (In contrast, carbohydrates — an important energy source for humans — are relatively unimportant in fish nutrition.) Vitamins and minerals, often in trace amounts, are essential for metabolic performance.
Dietary proteins provide the ten or so essential amino acids (and many of the non-essential amino acids) that fish require but cannot synthesize. The natural diet of fish is protein rich. Thus, the pondkeeper must ensure that protein requirements are met in captive feeding. Failure to do so is immediately obvious. A deficiency of even one of the essential amino acids results in a cessation of growth, and soon after the fish begin to waste away.
Protein Requirements for Koi and Goldfish as a Percentage of Daily Food Ration at 64 to 75
|Age Of Fish
||Percentage of Protein
|Up to 1 Year
|1 to 3 Years
|Over 3 Years
There are several factors that influence the precise protein requirements of koi and goldfish. Age is especially important. Young fish require a higher percentage of protein in their diet than older fish because the process of growth demands more amino acid absorption than does maintenance of the body (see Table I).
Water temperature affects protein requirements. When water temperatures are low — say, below 60 degrees Fahrenheit — growth is slow and protein demand is lower. Protein percentages of about 25 percent are appropriate for all ages of fish in cool waters.
The feeding rate also affects the protein requirement. If the quantity of food offered is less than satiation of the fish's appetite, then higher protein content is needed to meet total daily protein demand. If the food contains too much fiber or starch, net protein (and other nutrient) uptake is reduced. Again, higher protein content would be called for.
The amino acid content and digestibility of various proteins and protein sources differ greatly. Therefore, the exact protein source is important. Fish meal and soybean meal provide readily digestible proteins. Animal meats and corn meal, on the other hand, contain large fractions of undigestible proteins.
Dietary fat is the major energy source for fish. In the wild, the percentage of lipids in the daily diet varies from 10 to 40 percent (dry weight). But energy requirements depend greatly on activity level, and, in general, carp in the wild are much more active than those held in ornamental ponds. Thus, roughly 5 to 10 percent of a koi or goldfish diet should consist fatty acids.
In particular, carp require linolenic and linoleic fatty acids. Fish oils (e.g., cod liver oil, salmon oil) appear to be the best sources of essential fatty acids, averaging around 25 percent linolenic fatty acids but only around 2.5 percent linoleic fatty acids. In contrast, vegetable oils (e.g., soybean oil and corn oil) are fairly low in linolenic fatty acids (about 4 percent) but high in linoleic fatty acids (52 percent). Thus, a proper diet for koi and goldfish would contain about 1 percent (dry weight) of both types of oil. Linseed oil is a good compromise, containing sufficient quantities of both fatty acid types. (Saturated fats, such as animal fats, are not very useful to fish.)
Vitamins, often in very small quantities, are essential for good fish health. For example, thiamin deficiencies may be mistaken for organophosphate (insecticide) poisoning, both of which produce instability, equilibrium loss, body curvature and, ultimately, death. Biotin deficiencies can mimic external parasite infestations, with characteristic skin lesions, blue slime covering the scales, spastic convulsions and poor growth.
Basic vitamin requirements are listed in Table II. Carp can synthesize some vitamins, such as B12, reducing the need for dietary sources, but most vitamins must be obtained through the diet.
Basic Vitamin Requirements for Fish
||Quantity Per Kilogram Of Feed Per Day
|Ascorbic Acid (Vitamin C)
||500 - 3,000 mg
||100 - 150 mg
||40 - 50 mg
||10 - 20 mg
Minerals are also essential to fish health. They play a role in tissue formation and basic metabolic function — especially in maintaining the osmotic balance between fluids in the fish's body and the water. In fact, it is through osmotic diffusion that fish can satisfy many of their mineral needs, given that the water contains the correct trace minerals. For example, chloride, carbonate, sulfate, sodium, calcium, phosphorus, potassium and iodine can all be obtained from diffusion. In contrast, requirements for copper, magnesium, iron and zinc must be satisfied via dietary means.
Carbohydrates, as noted above, are not an important element in koi and goldfish diets. In fact, most of these ornamental fish suffer from diets too rich in carbohydrates. Severe liver degeneration, excessive glycogen (a starch) deposits on the liver, liver enlargement, and kidney and heart failure can result from the overfeeding of carbohydrates. A safe practice is to ensure that carbohydrate consumption remains below 10 percent of the daily diet.
Similarly, fish do not need fiber. When fiber (bulk) represents more than 10 percent of daily food intake, food evacuation through the digestive tract is accelerated. This reduces nutrient absorption in the intestinal tract significantly.
Koi and Goldfish Physiology
As noted at the beginning of this article, koi and goldfish can make use of a wide variety of food sources. In fact, their physiology has adapted for this very purpose. Koi and goldfish mouths have the ability to extend forward like a vacuum cleaner nozzle for scavenging pond and river bottoms for food. Koi have special "barbel" sensory organs at the corners of their mouths that enable them to discriminate among food types as they poke around a pond bottom.
Both koi and goldfish have small hollows located at the back of the gill arches that trap all kinds of suspended food-laden debris as water passes by. In the back of the throat, there are pharyngeal teeth for grinding up coarse matter. These ingestion mechanisms allow koi and goldfish to feed continuously on a variety of foods of fairly small size in relatively small quantities. They do not specialize in one type of food, nor are do they consume an entire meal in one big bite.
Neither goldfish nor koi have true stomachs, so they cannot hold large pieces of food internally. They also lack an "acid bath" — characteristic of animals with stomachs — for digesting food. Rather, they have long, folded, alkaline intestinal tracts where enzymes digest food matter all along the winding intestinal path. In other words, as with their ingestion mechanisms, the digestive tracts of koi and goldfish are designed for fairly continuous processing of small amounts of diverse food types.
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.
The Feeding Process
There are several factors that influence the appropriate frequency for feeding pond fish. Physiology, water temperature, the availability of natural foods in the pond, prepared food characteristics and fish load are probably the most important.
How Much Food?
The amount of food required to keep koi and goldfish healthy — assuming that the food has the nutritional characteristics already described — depends on two main factors: the age of the fish and the water temperature. Table III offers some rules of thumb for the total daily feeding requirement. It is intended to be a basic guideline — not a hard law of biology.
For example, a 12-inch, three-year-old koi might weigh 400 grams. During the summer, therefore, it should be fed roughly 4 grams of food (dry weight) daily. If, as discussed below, you break the feedings up into four offerings per day, then each offering will be one-fourth of this total daily ration: 1 gram.
The quantity of food should be reduced when water temperatures significantly exceed 78 degrees Fahrenheit for an extended period of time. In most instances the fish will let you know by not consuming all of the food. Uneaten food should be removed immediately after allowing five minutes of feeding time.
Calculating The Amount Of Food
The guidelines presented in this article are just that — guidelines. They can help you determine whether your feeding practices are reasonable. There are three things you need to know in figuring out the correct amount of food for each fish: 1) the age of the fish, 2) its mass, which is related to its length and 3) the amount of food as a percentage of the fish's mass.
Consider, for example, a pond that holds five one-year-old koi and three four-year-old koi. Suppose the one-year-old koi are about 5 inches long and the four-year-old koi are about 10 inches long. It is summer time and the pond water temperature is 72 degrees Fahrenheit.
We can estimate that a 5-inch fish weighs about 22 grams and a 10-inch fish weighs about 200 grams. Based on age, weight and water temperature (see Table III), each of the one-year olds should receive about 2 percent of its body weight as food per day, which is 0.44 grams (440 milligrams). Each of the four-year olds should receive about 1 percent per day, which is 2 grams of food. All totalled, the fish should be fed (5 x 0.44 =) 2.2 grams plus (3 x 2 =) 6 grams, which is 8.2 grams of base food per day.
Now, if we are feeding our fish four times daily, then each feeding would require about 2.15 grams for all the fish. It may be that the fish eat more in the afternoon than in the morning, but on average approximately 7 to 10 grams of food should be consumed per day. If the amount of food you use varies significantly from this, you should re-examine your feeding practices, the health of your fish or both.
How Often to Feed?
From the little that I've said about koi and goldfish physiology, it should be obvious that forcing these pond fish to consume one large meal each day will surely cause a variety of health problems. On the one hand, the more aggressive fish will get all the food, while the more timid fish will slowly suffer from malnutrition. On the other hand, those fish that do ingest large amounts of food will suffer a decline in their abilities to extract nutrients from the food. At the same time, they will be at greater risk of developing intestinal impaction — that is, food jamming in the turns of the intestine. This latter condition leads rapidly to internal aeromonad infections (usually manifested by dropsy).
Optimally, you would like to have a pond setup where the fish could just graze all day long at their own pace. A very lightly stocked garden pond with plenty of edible plants, algae, worms and insects is an ideal aquatic environment. Here, premium pelleted foods may be added as a nutritional supplement or for color enhancement.
In reality, the optimal case is rarely possible, and scheduled feedings are required to maintain adequate diet. As Table III shows, when the water is cold — in the low 50-degree Fahrenheit range or below — the fish should not be fed at all (although they may chose to graze on pond algae). When the water temperatures are in the mid 50- to mid 60-degree Fahrenheit range, two or three feedings that total the daily food ration will suffice.
As the water temperature rises into the high 60-degree Fahrenheit range and above, I recommend four and preferably more feedings per day. If there is some natural food in your pond — aquatic plants, small invertebrates and so on — then the total daily ration can be cut, but the number of feedings should stay the same. Remember, you are trying to replicate the continuous feeding process of carp and goldfish, not just the total daily ingestion of food.
Total Daily Food Quantity for Koi and Goldfish as a Percentage of Fish Body Mass*
||Below 55 °F
||55 - 64 °F
||64 - 77 °F
1 - 3%
|3 - 5%
Up to 1 year
||0.5 - 1%
||2 - 3%
1 to 3 years
||0.5 - 1%
||1 - 2%
Over 3 years
||0.5 - 1%
*Percentage of food is for dry weight and assumes correct basic ingredient characteristics
I would like to note an important, but sometimes misunderstood, aspect of the well-known "five minute" rule. This rule states: let your fish have all they can eat in five minutes, but no more. The point to remember is that this rule applies to each feeding, not to the total amount of food for the day.
I visited two ponds last summer in which the fish were practically starved to death. In both cases, the owners said they were letting the fish eat all they could for five minutes a day, then removing the leftovers. If you feed your fish only once a day in the middle of the summer, the five minute rule will ensure malnutrition! The proper procedure during the summer would be five feedings, each five minutes long.
Some Final Thoughts
There are several additional points that are important to keep in mind. First, the shelf life of fish food is not infinite, even if it looks and smells okay. In particular, fats and vitamins break down fairly quickly upon exposure to heat and air. The nutrient assay of any fish food pertains only to fresh food, not food that has been lying around the garage for six months.
In general, fish food should always be kept in air- and light-tight containers in cool locations (60 degrees Fahrenheit or less). To ensure freshness, I suggest that you not buy more than a month's requirement at a time. I keep all but enough for one week in cold storage.
Second, unless you have an excellent aeration system and you have precisely measured the dissolved oxygen levels in your backyard pond during the night and morning, you should avoid feeding your pond fish in early morning or late evening. As the plants and algae switch to respiration during the night, dissolved oxygen levels in a pond drop significantly. Fish oxygen requirements increase several times above the normal level during and immediately after feeding. Thus, feeding late in the evening or early in the morning runs the risk that your fish will have an increased need for oxygen at the time when dissolved oxygen is at its lowest levels in the pond. As a guide, I suggest that you not feed until sunlight has been on the pond for at least two hours, and not within two hours of sunset.
Third, water changes represent an important element of proper koi and goldfish nutrition. As noted earlier, they can absorb many essential minerals from the water. As time goes on, however, the dissolved mineral content in the pond water decreases and it must be replenished. If the level of minerals in the water drops too low over time, the fish can actually suffer mineral depletion. (Incidently, this is why keeping fish in distilled water will eventually kill them.)
Lastly, breakfast cereals are not appropriate as either base or supplemental foods. Regardless of what you have read elsewhere, feeding such foods will be appreciated only by your grocer, not your fish.V This article is certainly not the last work on feeding koi and goldfish. I hope, however, it is a good start toward helping you understand the feeding requirements of your pond fish.