The label on fish food containers will list the ingredients that are used to make the food. They are listed in order of highest concentration in the food first. Look for food that has the first few ingredients listed as fish, shrimp, or other seafood for carnivorous fishes, and algae or vegetables for herbivores. There should be minimal amounts of grains used in aquarium fish food.

The label should also contain a Guaranteed Analysis, which the lists the percentages of protein, fat, carbohydrate, fiber, vitamins and minerals in the food. A good quality food contains a high percentage of digestible protein, plus essential amino acids and fatty acids, vitamins and minerals.

The price of food mainly depends on its ingredients. It is cheaper to produce food with fishmeal as the main protein, and a high content of cereals and a low content of fish oils, than it is to produce high quality food using fresh fish or other whole seafood (such as shrimp, squid, clams, krill) as main ingredients. But the price of a high quality food does not necessarily mean that feeding fish is more expensive, as the amount of food to be used may be less as it is more digestible (digestibility is the amount of food that is assimilated by the organism and is not eliminated as waste through excrements). The digestibility of carbohydrates by fish is only 34 percent, compared to proteins and fats fat at 85-95 percent digestibility. This means that you will use less of a food with high energy value and digestibility (higher in protein and fat) compared to foods with more carbohydrate (from grains or vegetables).

Fish diets should be low in fat. Even meat-eating fish (carnivores) require a limit of no more than 8-10 percent fat in their diet. Plant eaters (herbivores) need no more than 3-5 percent fat. Excessive fat will damage the liver​ and can result in disease and early death.

The type of fat also matters, as fish have difficulties digesting hard (saturated) fats, such as those in beef. These saturated fats are particularly harmful and should be avoided. Polyunsaturated fats (oils), such as those in brine shrimp, are the most digestible​ and are particularly useful when conditioning fish for breeding. It is important that the food contains the essential Omega-3 and Omega-6 fatty acids for healthy fish growth.

Fish do not need carbohydrates in their diet. In fact, too many carbs can deter proper growth, as fish are not able to readily digest carbohydrates like land animals do. However, there is variation by species to the amount of carbohydrate a fish can tolerate without suffering negative side effects.

Perhaps the greatest danger in feeding higher percentages of carbs is the resulting reduction in all of the other essential nutrients available in the diet. This is particularly true in young fish, which need high levels of protein for proper development. Adult fish, however, can tolerate as much as 40 percent carbohydrate in their diet, seemingly without ill effects, although 25 percent is better. Most of the carbohydrate in fish food comes in the form of starches (from grains) that are used to bind the food and prevent it from rapidly disintegrating in water.

Fiber is the non-digestible form of carbohydrate (cellulose and lignin). Although small quantities of fiber are important in the diet to aid in digestion, they should not be too high. Carnivores are not able to digest fiber well, and should not have more than 4 percent fiber in their diet. To remain healthy, herbivorous fish should have between 5 percent and 10 percent fiber in their diet.

Protein requirements vary widely based on the species of fish. Good quality protein is the most expensive part of the components in fish food. However, protein is a key element required for good health and growth in all fish species. Herbivores need 15 percent to 30 percent protein in their diet, while carnivores need at least 45 percent protein. For vigorous, healthy growth, young fish require a diet that is composed of at least 50 percent protein.

Minerals are important for healthy cells, immune systems, metabolic enzymes, bones, teeth, and even for maintaining healthy scales. The key minerals fish need in bulk are calcium and phosphorus. Calcium is found in hard water and can be absorbed through the gills, and phosphorus is found in live underwater plants. Fish also need trace amounts of iron, iodine, magnesium, sodium, chloride, potassium, sulfur, copper, and zinc.

If the aquarium water is soft (mineral poor) and the tank decorated with only artificial plants, it is important to supplement the diet with mineral containing foods. Bone or meat meal is a good source of both calcium and phosphorus, as well. Minerals have a long shelf life and can be found in adequate quantities in all good quality pelleted and flake foods.​

Unlike minerals, vitamins are not stable for very long in prepared foods. Flake foods have adequate vitamin content initially, but the content degrades (oxidizes) rather quickly once the container is opened and exposed to air. Storage in the refrigerator or freezer will prolong the stability of the vitamin content, however, it is best to buy only what you will use within a month.

Key vitamins needed for good health are A, D3, E, K, B1, B2, B3, B5, B6, B12, Biotin, Choline, Folacin and Inositol. Vitamin C (Ascorbic Acid) is important for its anti-oxidant and anti-inflammatory activity, but has a short (6-month) shelf life. Find food with stabilized vitamin C (L-ascorbyl-2-polyphosphate) that has a longer shelf life.

Many fish keepers are not aware of the critical role vitamins play in fish health. Lack of vitamin A can cause spinal deformities and stunted growth in young developing fish. Anytime a fish is under stress, the need for vitamin A is increased, which can mean the difference between falling prey to disease and remaining healthy. Vitamins E and A are key factors in maintaining fish in top breeding condition. Vitamin K is critical for proper blood clotting.

Vitamins B1, B2, and B6 are important for normal growth. Good digestion requires an adequate amount of vitamins B3 and C. Vitamin C is also needed for healthy bones and teeth, which are important in all species of fish. Both vitamins B5 and Inositol are key factors in metabolism. Lack of Biotin and Folacin reduces the formation of blood cells and can cause anemia.

Purchasing all foods in small quantities, and varying the diet using an array of good quality dry and frozen foods are the best ways to promote good nutrition. Adding in enriched (well-fed) live foods will help assure that your fish get all the nutrients they need for good health and long life.

One further note of importance about feeding is how much to feed. If you feed your fish cheap, inadequate food, they will need to be fed a large quantity of this poor quality food. Some people think that fish are always hungry because they are begging for food. This may indicate they are not getting proper nutrition.

Please, feed your fish two times a day with a high quality food. Give them as much food as they will eat in about 5 minutes, with no leftover food on the bottom. If you have bottom feeding fish, you may also want to use some sinking pellets for them to ensure they get the food also.