 Page 4 of 11 Protein - The Building Blocks:
Many hobbyists seem to get hung up on the protein percentages that are shown on fish food labels, without truly understanding exactly what those numbers mean.
High quality protein that is easily digested by fish does not cause gastrointestinal issues, no matter if the fish is classified as herbivorous, omnivorous, or carnivorous. Excess protein is for the most part excreted as waste. The idea of excess protein causing bloat, or any type of gastrointestinal issues in herbivores, is totally inaccurate. Poor quality protein, as well as other hard to digest ingredients, can cause bloat, etc., but easily digestible protein will never cause any dietary issues. Hobbyists that keep carnivores want the highest protein percentage in a food that they can find, those that keep herbivores seem to want the least, with neither of them understanding the basic principles involved. My advice is to forget about the numbers posted on a label, and concentrate on the quality of the protein.
Many people think of Fish meal as being a poor quality source of protein. This is simply not true. Fish meal is used in a wide variety of animal feed applications including pet food, poultry, and protein blends. Fish meal is an excellent source of protein and is rich in essential amino acids, fats, and vitamins and minerals. High quality Fish meals such as Herring meal are processed from whole fish, not processing plant by-products. Certain types of lower quality meals are most likely the cause for Fish meal getting a "bad rap" over the years, and if the Fish meal is comprised of processing plant by-products such as heads, scales, and bones,it will usually result in excessive ash content in the final product. Some fish food labels do not even list the maximum ash content. Obviously it is also much less costly for a manufacturer to use these types of raw ingredients, compared to a high quality source of marine protein such as South Antarctic Krill, Herring, Mussel, or Squid. On the other hand, if the Fish meal is not listed in the first ingredient and the ash content is less than 9%, it usually indicates that too much Soybean, Corn gluten, or Blood meal is being used. While some fish such as Koi can assimilate large amounts of Soybean meal and Corn gluten, most tropical species cannot. Blood meal, though high in protein, is low in many essential amino acids. Also keep in mind that a green colored food does not necessarily contain a large quantity of vegetable matter, any more than a red colored food equates to Krill being used as the primary ingredient.
Carbohydrates: |
The protein percent on a fish food label doesn't really tell you anything about the quality of the protein. The value of protein is directly related to the amino acid content, such as Arginine, Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Trytophan, and Valine, which essentially are the building blocks for muscle and growth. The protein percent shown on a label doesn't tell you how that protein was processed, or if it's even in a form that your fish can properly digest and utilize. Unless that protein can be fully digested by your fish, the crude protein percent on a label becomes somewhat meaningless. 
The ingredients listed on a food label are your first clue as to the quality of the food's protein content. Avoid those that contain too much grain such as wheat middlings, corn, brans, flour, potatoes, or protein derived from Soybean meal. (As indicated by the first ingredients listed; the ingredients that are listed first are the most prevalent ingredients). Look instead for foods with high-quality, marine-based proteins such as whole Herring, Krill, Mussel, and Squid at the top of their ingredient lists. 
Fish do not use carbohydrates very efficiently, and in quality feeds their use is primarily as a binding agent during the manufacturing process. Without some grain the food would simply not hold together. Due to the fact that carbohydrates can be used as a rather inexpensive source of energy in fish foods, some foods utilize a high amount of grains in their formulas to reduce feed costs. The logic is, if it's cheaper, then why not? Grains do have their place in fish foods, serving as binders and to help synthesize lipids and protein. However, if excessive amounts are used, those excess amounts can get stored as fat and also will increase the total amount of undigested solid waste being expelled by the fish. This leads to unwanted pollution being added to your aquarium. Grain by-products also are very difficult to digest by many species, and when used excessively can cause gastrointestinal issues due to poor digestibility and absorption rates. Consequently, pathogenic bacteria start multiplying inside the intestinal tracts, doubling their population as quickly as every 20 minutes, resulting in bloat. In most cases this condition is extremely contagious and the end result usually is fatal.