Proper and balanced nutrition.
Macronutrients. About 99% of body mass is usually accounted for by five chemical elements – hydrogen, oxygen, carbon, nitrogen, and calcium. They participate in most chemical reactions occurring in the organism; they are part of proteins, fats, and carbohydrates. Another six elements – phosphorus, chlorine, potassium, sodium, sulfur, and magnesium together make up approximately 1% of the biomass of living beings. Problems with mineral metabolism are possible, for example, when keeping fish in excessively soft water. Food for soft-water fish should contain a greater amount of mineral substances than for others.
Micronutrients play various roles in the organism. Micronutrients affect growth, reproduction, hematopoiesis, etc. The organism’s need for micronutrients, as their name implies, is very low, but the organism cannot exist without them. There are over 30 micronutrients – metals and non-metals.
Minerals are absolutely necessary for skeletal formation and healthy fish growth (calcium and phosphorus). A fundamental difference between aquarium fishes and invertebrates compared to terrestrial animals is that mineral substances and vitamins can enter their organism not only with food or drink, but also directly from the surrounding water – through the gills and body integument. Taking advantage of this, aquarists often solve the problem of providing their pets with these substances by adding them not only to feeds, but also directly into the aquarium water. Fish living in hard water can extract calcium from the water, but fish living in soft water need a greater amount of calcium in their diet. Among other important elements, manganese, iron, zinc, etc. can be noted. They are needed in very small quantities. Usually, plant-eating fish receive these minerals along with plants and algae.
“Vitamins” are essential components for biochemical reactions. This is a single term for completely diverse substances. Accordingly, the action of various vitamins 
is very diverse, and the requirement for these vitamins also varies greatly. They participate in almost all biochemical and physiological processes that make up metabolism as a whole. Vitamins are required by organisms in very small quantities. Some feed components contain vitamins, but they are usually added as additional components. This is especially important for aquarium fish that do not receive live food and do not supplement their menu with plant food.
Water-soluble and fat-soluble vitamins are distinguished. In addition, a group of vitamin-like compounds is isolated. Vitamins such as A, D, E, K are classified as water-insoluble (they dissolve in oil) and therefore must be added together with food. Water-soluble ones include: ascorbic acid (vitamin C), B vitamins – thiamine, riboflavin, (cobalamin), niacin (vitamin PP), folacin, pantothenic acid, biotin, and they can be added as special supplements. The absence of any vitamin leads to slowed growth, weakened immune system, etc. The deficiency of one vitamin cannot be compensated by another.
Acute avitaminosis is usually the result of abundant and monotonous feeding or the use of medications. At the same time, it should not be forgotten that even in cases where obvious symptoms of avitaminosis are not observed, the introduction of special aquarium vitamin preparations significantly improves well-being, increases immunity, and enhances the coloration of many fish and invertebrates. In addition, these preparations help them survive difficult situations associated with stress, reproduction, the use of medications, etc.
Pigments (colorants). Pigments contained in feed help improve the coloration of aquarium fishes. Pigments (carotenes) are responsible for the yellow, red, orange, green coloration of fish and invertebrates. Bright blue and purple colors are also caused by carotenes, although they may have another source. In addition to coloration, these pigments are necessary nutrients for the development of tissues, caviar, etc. Live feeds such as *Artemia* and other crustaceans are especially rich in these pigments. Also, pigments (carotenes, xanthophylls) are added to artificial feeds.
Live and fresh feeds (less frozen) are more complete than substitutes – compound feeds, mixtures, etc. All substitutes for live feeds are incomplete in their composition and cannot replace live feed, so using them for a long time is not worthwhile. The most useful natural live foods for fish are: bloodworms, *Corethra*, tubifex, cladoceran crustaceans (*Daphnia*), copepod crustaceans – *Cyclops*. Aquatic feed organisms should be taken from natural water bodies where there are no fish, otherwise they may contain parasites that cause fish diseases.
The vast majority of fish species are satisfied with 30-40% protein in their feed, and fishmeal proteins contain a growth factor and significantly surpass any others in nutritional value. Essential amino acids for them will be: arginine, histidine, isoleucine, leucine, valine, lysine, phenylalanine, threonine, and tryptophan. Essential substances also include choline, cephalin, and iodine. A deficiency of almost every amino acid causes a loss of appetite in fish, manifested in poor food consumption and a decrease in growth rates. Adding polyamine to the water can help.
The main non-protein energy source is fats. If they are in the feed in a liquid state (linolenic acid), they are well absorbed. Calcium and phosphorus are adsorbed by the skin and gills directly from the water.
Vitamins come mainly with food (crustaceans are the most vitamin-rich). The daily requirement for vitamins looks approximately like this (per 1 kg of fish weight): retinol (A) – 100-500 mg, thiamine (B1) – 0.15 mg, riboflavin (B2) – 0.11-0.33 mg, pantothenic acid (B3) – 0.1-1.4 mg, nicotinamide – 0.55 mg, etc. A deficiency of microelements, in particular iron, leads to anemia, scoliosis, skull deformities, and convulsions.
Energy requirements in fish, unlike warm-blooded animals that have significant costs to maintain a stable body temperature, are small. Since all biological systems obey the laws of thermodynamics, energy absorbed with food is converted into one form or another – expended in metabolism, stored during growth, or excreted with feces. In herbivorous fish, energy losses with feces account for 40-50%, in omnivorous fish 25-30%, in carnivorous fish up to 20%.
The digestibility of animal food is 84-96%, plant food 80-90%, detritus 10-15%, etc. Casein is consumed practically without remainder, at 99%, beef heart at 96%, squids and polychaetes at 95.5, fish protein concentrate at 94, opossum shrimp (*Mysis*) at 83.8, amphipods at 72.7, tubifex at 71-85, zebra mussel (*Dreissena*) at 28.3%, etc. Feeds with an increased protein and fat content are digested slower.
Basic principles of feeding aquarium fish.
