Fish nutrition

The nutrition of fishes isn't that alien. After all, fishes are our own stay-at-home cousins. Their main digestive enzyme, like ours, is pepsin. Like all vertebrates they can't manufacture all the amino acids they require: they have to collect them here and there in their diet. Yet all the amino acids must be present for protein synthesis; lack of any one becomes a "limiting factor" that puts a cap on the usefulness of all the others. This is the fundamental reason for varying your fishes' diet.
 
Protein and lipids. Growing fry require more protein than adult fish. And small fish require more protein than large fish. In natural conditions, fish diet is largely protein and lipids (fats and oils). Though fishes require lipids, there is little carbohydrate in a natural fish diet. Any cereals and cereal by-products in flake feeds are essentially fillers — the cheapest ingredients in processed feeds.
 
Fish metabolism. Comparisons of the actual caloric energy that fish realize from metabolism to the theoretical possible maximum suggests that the efficiency of fish metabolism lies slightly under 50%. The rest is lost, mostly as muscle heat dissipated in the water. Factors that influence the nutrient requirements of fish include low energy requirements, which are strongly affected by temperature. Adult carnivorous fishes require 40 to 55% dietary protein, while omnivores need only 35 to 45%. As fishes mature, they utilize less of their available dietary protein. And at all stages of their lives, high-intensity feeding leads to less efficient protein conversion. In other words, though excess protein goes in, it can't be assimilated and passes right through.
 
As you know, fishes are poikilothermic; their metabolism varies with the water temperature. At lower temperatures, metabolism slows and less food is required.
 
Phosphate. Two-thirds of the phosphate found in plants and half the animal phosphate passes right through a fishes' system to fuel the phosphate cycle in your aquarium. Ordinarily fish get all the calcium they need in their carnivorous diets, but when they were experimentally deprived of calcium, channel catfish proved efficient at absorbing it across their gills, even from water that was as low in calcium as 5ppm.
 
Vitamins are absorbed from the fishes' intestine. Water-soluble vitamins are constantly excreted, but fat-soluble vitamins may be stored, and excesses can cause trouble.  Experiments in 2006 to detect any difference in pigmentation —  or survival, growth or antioxidant capacity —  of dietary beta-carotein or astaxanthin  on the already rosy Hyphessobrycon callistus, one of  the "Serpa Tetras", could find none; then experiments in 2008 on the "Flame Gourami" variety of Colisa lalia, did find an effect, both directly observed and in the reaction of females.
 
Fishkeepers often feed some lightly boiled liver or beef heart to larger, carnivorous fish. Beef heart provides concentrated protein to maximize the growth of fry, so discus breeders use it. But for the average adult fish in captivity, there's too much protein in heart and liver to make it a steady diet.
 
There's a risk involved here, though healthy mammal liver tissue and heart muscle don't have fat in them. Mammal and bird fats tend to be unsaturated fats. In the warm-blooded creature they remain soft, but at cool fish temperatures they harden and go waxy. So, though you may feed some beef heart or liver, muscle meat from any bird or mammal is not recommended as fish food. The essential unsaturated fatty acids that must be supplied in the fishes' diet, are not the same series that warm-blooded creatures require. "Wild Discovery" photographers might throw the carcass of a capybara to the piranhas, but the piranhas' more usual prey are other fish. Fish aren't geared to deal with much fat and lipids anyway. Surplus fats that can't be burned get stored in the fishes' liver. Eventually the liver tends to enlarge, finally badly enough to make the fish look bloated. Liver disfunction results in retained water, a condition we still might be calling "dropsy" or attributing to constipation.
 
When you read about fish nutrition in the documents linked below, keep in mind a basic difference in focus. Fish nutritionists are essentially writing for aquaculturists. They are trying to maximize rapid weight gain for food fish. Though in these cases nutritionists will recommend carbohydrates as a cheap source of energy, they won't be recommending those carbohydrates that aren't easily digestible by fish, which include cellulose and hemicellulose, gums and pectins — all forms of dietary fiber. Your objectives are somewhat different: you are feeding to maintain fish that remain lean and active into a ripe old age.
 
Links. "Fish nutrition: choosing a basic freshwater fish diet" at Drs Foster & Smith Pet Education, D.F. Bobo, "The basics of fish nutrition" at Age of Aquariums  and Pablo Tepoot, "Basic fish nutrition" at Wet Web Media all give you sound basics.
 
Adrian Tappin's introduction to feeding rainbowfishes applies to all the omnivores.
 
Don Johnson's brief 2001 hobbyist-oriented article "Fish nutrition" is archived at FishGeeks.
 
 The FAO document "Fish feed technology" is a U.N. handbook covering every aspect of nutrition and digestion in teleost fishes, slanted towards aquaculture of food fishes. If you have a geeky streak, you'll enjoy it.