I want to start with a disclaimer; most of the information I have is technically unsubstantiated. Most fish nutritional research work has been focused on the more economically important salmonids like trout and salmon. Food varieties of carp have received some attention, but it is probably risky to generalize these results to goldfish without some consideration. So take this section as informational and please pass along any facts or insights you may have!
Goldfish seem to be pretty flexible with respect to diet. They can survive for very long periods on remarkably extreme diets. However, my interest has been in finding an optimal diet, not an adequate diet. The first question to ask is "why not just use commercial goldfish food?" and let someone else worry about all this. Well, there are several reasons, not the least of which is that I find this an interesting subject that I enjoy spending time investigating. I'm getting smarter about what my goldfish need, but I've also changed my own diet as a result of what I've learned.
There are some excellent prepared foods on the market. I prefer to feed a sinking food (so that my fish don't gulp air while eating; some fish have floating troubles due to this), which limits me to a pellet-type prepared food. The general wisdom seems to be that if more than one or two pellets are given in a feeding, they should be presoaked for 10 minutes, so they become waterlogged. This is to avoid the pellets becoming impacted in the fish's gut as they absorb water. Unfortunately, there is substantial evidence that some of the nutrients (particularly soluble vitamins) leach out very quickly during the soaking process. This is especially true for flake foods.
Other issues for prepared foods are the decreasing nutritive value with age and the quality of the ingredients. The latter issue is just an economical one; even the best intentioned manufacturer cannot use the choicest ingredients and still make a profit. So they must find the best ingredients that are economically feasible. Whether my perception of a "choice ingredient" has any bearing on the health of my goldfish is of course debatable.
The alternative is to make your own food. This idea leads to the recipes in another section and to this discussion about nutrition. For my fish, I feed a homemade gelatin food supplemented regularly with commercial pellets.
Fish food labels usually provide a fairly crude nutritional analysis, limited to a breakdown of the percent protein, fat, and fiber. This is a good starting point, though for goldfish carbohydrates probably should also be considered. Animals use the foods they eat for growth, repair, and for energy. Protein, fats, and carbohydrates can all be used for energy. However, for building and repairing specific types of bodily components, these nutrients are less interchangeable, hence the importance of a "balanced diet". Let's look at protein, carbohydrates, fats, and fiber in more detail with respect to goldfish.
From reading and observing what more experienced fanciers do, I would conjecture that goldfish seem to do best on a fairly high protein diet, similar to that recommended for Koi and food carp. This was surprising to me. However, realize that goldfish needs vary with age and the season of the year. You can't feed the same diet from birth to death, and get good results.
Rough ranges seem to be (in percentage of dry weight):
These figures are higher than what Post recommends for carp, but are closer to what I have observed other goldfish fanciers doing. Note that this is on a dried basis! Dried spinach is about 25% protein... dried spirulina algae is 60%! So even a standard mixture of one part green peas, one part spinach, and one part fish is, on a dry basis, over 50% protein. Most prepared food labels include 10-15% moisture as an ingredient, which means they would report a protein level that is 10-15% lower than my "dry basis" for exactly the same recipe.
What happens outside these ranges? Assumedly too little protein will lead to slow or stunted growth, difficulty in conditioning for breeding, and probably a host of other problems. To much protein will lead to a lot of ammonia and partially digested protein wastes, and, if the human case can be used as an analogy, possible kidney problems over the long term, though I would say that many very successful goldfish keepers feed mixes at the high ends of these ranges. Long, stringy feces have also been cited as a symptom of too much protein. I could easily believe that too little roughage might be as much of a factor.
Beyond the percentage of protein, the quality of the protein and the availability of critical amino acids (the building blocks of proteins) are big concerns. Some goldfish fanciers add amino acid supplements to their homemade foods. The reasoning is that goldfish have fairly simple digestive systems and may have trouble extracting critical amino acids from complex protein molecules. I'm a little skeptical of this, but I'm sure it wouldn't hurt as long as appropriate amino acids are added.
The quality of a protein source for a given animal is determined by comparing both the quantity and availability of essential amino acids in the proteins in the food source to the actual requirements of the animal. The closer the match, the higher quality the protein. For a long time nutritionists called chicken eggs a perfect protein source for humans and used them as a reference for comparison to other sources. This is no longer true, but eggs are still considered a very high quality protein source for humans. Some goldfish fanciers also swear by them for their fish.
For reference, Post offers a table of the critical amino acids for carp. Again, there are no assurances that this can be applied directly to goldfish, but it seems like a reasonable starting point. These amino acids are:
Arginine, Cystine, Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, and Valine
The only use I would suggest making of this list is to not knowingly feed a diet deficient in any of these amino acids. It is worth noting that gelatin, which is a significant source of protein in some of the gel food recipes, lacks cystine, isoleucine, threonine, tryptophan, and valine. And, based on the list in Post, more than 75% of the content of gelatin is non-essential amino acids. So without even looking at percentages, gelatin is far from a perfect protein for fish. Unfortunately, my experiments with other binders (like the carbohydrate-based agar-agar) have not turned up a substitute that satisfactorily survives freezing.
I have more work to do before I can give a list of good protein sources. Some people have suggested to me that goldfish do better if they are provided a diet with animal protein sources in addition to plant sources.
It seems well accepted that carbohydrates are converted by goldfish to fat deposits in the fall to support winter dormancy. Also, full round-bodied fancy goldfish will not develop properly deep bodies without sufficient carbohydrates. So it seems clear that carbohydrates need to be a significant component of the goldfish diet.
Post talks about trout utilization of carbohydrates: the net is that they can only digest simple sugars and more than 20% digestible carbohydrate can mess up their metabolisms. Since trout are exclusively carnivorous, this is not surprising. Carp in general, and goldfish in particular, are much more omnivorous, so it reasonable to expect they are better equipped to handle the natural carbohydrate content of plant matter. But their digestive tracks are nowhere near as complex as, say, a cow. So it is questionable whether they can utilize extremely complex carbohydrates like cellulose.
One thing that is true for humans and trout, and probably true for goldfish, is that the digestibility of starches are dramatically improved (often nearly doubled) by cooking. So if your food has a starch component (like rice), cooking it is probably a good idea.
I noted that Bob Mertlich in one of his fish food recipes adds some table sugar (sucrose) to his gel food. I haven't added sugar to any of the food I've made, but the plant matter in the recipes has a fair amount of carbohydrates. Green peas, for example, contain significant amounts of simple and complex carbohydrates. The fish seem to digest peas well, so I infer that carbohydrates are certainly an appropriate major component of the goldfish diet.
Fish require fat in their diet as an energy source and also as a critical building block for certain physical structures. When I make my gelatin food, I have been adding some fat to it in the form of an "essential fatty acid" supplement. I also use fish in the mixture that contains significant fat. Post discusses commercial carp foods as having as much as 10% dietary fats. I have aimed for a fat total in my gelatin food that is around this level.
Beyond the percentage of fat in the diet, there are specific fatty acids that essential elements in the diet. These are the omega fatty acids that we've heard so much about being good for people. Just like we can get them from eating fish, so can goldfish. Post talks about two being essential for carp, linolenic and linoleic fatty acids, with optimal growth results occurring when each is about 1% of the diet. These can be introduced into the diet through the use of fishes high in omega fatty acids (king mackerel and rainbow smelt, as examples), added fish oils, or even plant-based fatty acid supplements (grapeseed and borage oils are good sources, as examples).
One caveat; oils turn rancid through oxidation, and the products of rancidification are toxic. Unfortunately, freezing a batch of purchased or homemade food may slow, but will not halt, this process. I have yet to fully deal with this aspect. The best suggestion I can think of is to use oil products that are stabilized chemically for any added fats in the recipe. Commercial fish foods I assume are usually treated this way.
I don't have too much to say on fiber. In general, goldfish seem to have fewer digestion problems when they are fed vegetable roughage on a regular basis. It is possible that goldfish may get some nutritional value from cellulose, which is a very complex carbohydrate and a principle type of dietary bulk fiber, but I doubt it. I think the chief usage of fiber is in preventing the impaction of foods in the digestive track.
However, as with people, a very high fiber diet may reduce the availability of certain vitamins and minerals. It may also interfere with the digestibility of an otherwise balanced food source. My best guess is that a good strategy is to feed a primary food source that is balanced with respect to the principle nutrients (most homemade recipes are half to three quarters vegetables by volume anyway, so they contain significant bulk fiber), and then offer the fish supplementary roughage in the form of aquatic plants and leaf greens.
Post offers tables of mineral and vitamin requirements for fish. The March 1997 issue of Practical Fishkeeping also has a good discussion. The net is that fish, like higher animals, require a pretty complete assortment. It is worth noting that not all fish have the same needs; carp are cited as needing vitamin C, for example, while some other fish species can synthesize this nutrient and don't need it in their diets.
I have addressed the vitamin and mineral content when preparing goldfish food recipes by adding a vitamin and mineral supplement to the mixture and by trying to avoid excessive cooking that might destroy water soluble or easily oxidized vitamins. One strength of the gel food type of ration, I believe, is that is keeps these nutrients in the food until they are inside the fish. This is a reservation I have about the long-term feeding of soaked flakes or pellets to goldfish. While this may avoid impactions in the digestive track, the soaking is almost certainly removing significant amounts of the water soluble vitamins and minerals. An interesting way to combat this problem is to fortify the soaking water with a vitamin supplement (there are a number specifically formulated for aquarium fish).
After all this discussion, you would expect that you could pretty well account for everything that is in batch of fish food. When I make my own recipes, I can: I prepare them in a spreadsheet with full nutritional analysis of each ingredient. Based on this analysis, I usually see something like 40% protein, 44% carbohydrate (of which 14% is bulk fiber), 10% fats, and 6% ash (minerals and other non-combustible elements). Notice that this totals 100%; all the contents of the ration are accounted for by this nutritional analysis.
If you pick up a commercial food container, it will usually only account for 70-80% of the contents on the labeling. This may be because they simply leave out a nutrient like "carbohydrates". But it is hard to know. This question came to a head, for me, after reading an otherwise interesting column in a leading aquarium magazine. The column recommended (for goldfish more than 3 years old) 30% protein, less than 10% carbohydrate, 10% or less bulk fiber, and 10% fat. This, of course, adds up to 60%, leaving 40% of the content unaccounted for. I enjoy these columns very much, but in this particular case I just couldn't figure how to make any use of this guidance.
I have seen this type of analysis elsewhere. In general, it seems appropriate that recommendations of specific nutrient levels in a diet should include all the main elements. So if someone tells you that you are feeding "too much protein", they also need to tell you what you should feed instead (like "more carbohydrates").
I have repeatedly mentioned Post in this section; he is one source of basic information. Aquariology also has an interesting discussion. Two books I have not seen that are purported to have quite a bit of good information on fish nutrition are:
Stroskopf, Michael. Fish Health. (Don't have the complete reference, but for nutrition this is no better than Post and much more expensive)
Halver, J. E. 1972. Fish Nutrition. Academic Press, NYC, NY. 713p.
This last book, Halver, is essentially the authoritative reference. But, at $300 a copy, I'm looking to borrow this one.
Two periodicals that have some interesting information:
Aquarium Fish Magazine, April 1997 - A special issue on fish feeding.
Practical Fishkeeping, January, February, and March 1997 - A 3 part article on fish nutrition (this is a British publication: they can be reached at Tower Publishing Services, Tower House, Sovereign Park, Market Harborough LE16 9EF, UK)