Like humans, Fishes need all basic categories of nutrients to lead a long and healthy life. Carnivores are meat eaters and rely on a protein-heavy diet. Herbivores are plant eaters with digestive systems designed to extract nutrients from plant material. Most of the aquarium fishes are omnivores, which can eat both plants or meat or both.

To ensure that your fish stays fit and energetic, it is important to understand what and how to feed your fish, the daily nutritional requirements of your fish, and how these nutritional requirements vary according to various parameters like type, age, size, lifecycle (small, pregnant or nursing, adult), environment, physical activity, lifestyle, etc. Dietary deficiencies will not only shorten the lifespan of fish and cause many diseases, but will also contribute to a deteriorating water quality by polluting the water.

Food and Nutrition for Fishes

Many commercial foods may confuse the pet owners about the nutritional content of the food hence, it is important to know what foods your fish needs to stay healthy before interpreting the package information. Fishes also have varying feeding habits. Generally, three feeding groups can be identified-bottom feeders, mid-water feeders, and top or surface feeders. So based on your fish’s feeding preference and habits, one needs to select the food. Also, information about live foods is also important as many living flies, crickets, and worms do not contain any nutritional value. Live foods themselves must be fed a good diet if they are to be a complete source of nutrition for carnivorous fish.

There are 6 Nutrient Groups that are important for all fishes-proteins, carbohydrates, fats, fiber, vitamins, and minerals. The function of each of these is as mentioned below:

  1. Fats: Fishes require low amounts of fats in their diet. Carnivores require a limit of no more than 8-10 percent fat in their diet while Herbivores need no more than 3-5 percent fat. Excessive fat will damage the liver​ and can result in disease and early death. Fishes have difficulty in digesting saturated fats like those in beef while polyunsaturated oils like those in brine shrimps are digestible. Fish food should also contain Omega 3 and Omega 6 for healthy growth.
  2. Carbohydrates: Fishes do not need carbs in their diet. Adult fish can tolerate as much as 40 percent carbohydrate in their diet, while young ones can’t tolerate any carbohydrates and they need more proteins for development. 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). Only a small quantity of fiber is required in a fish’s diet to aid digestion. For Carnivores, it should be at most 4% while for herbivores, it should be between 5%-10% of their diet.
  3. Proteins: Protein is a key ingredient of all fish foods. 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.
  4. Minerals: Minerals are important for healthy cells, immune systems, metabolic enzymes, bones, teeth, and even for maintaining healthy scales. Fishes need Calcium and Phosphorus in bulk in their diet. 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.​
  5. Vitamins: Vitamins play a key role in a fish’s health. Key vitamins needed for good health are A, D3, E, K, B1, B2, B3, B5, B6, B12, Biotin, Choline, Folacin, and Inositol. One should consider the presence of all vitamins in the fish food before making the purchase. 

The label on fish food containers will indicate the ingredients that are used to make the food. One should look for ingredients like 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. 

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. 

There are a number of options of fish food to choose from as mentioned below:

Dry Food: The most common form of dried food includes flakes, granules, pellets that are available in both sinking and floating varieties. Dry food is lower in fiber hence adding vegetable foods to the diet will help reduce the risk of swim bladder disorders and bloating for vegetarian species.

 Frozen Food: Frozen food like shrimp, bloodworms, prawns, krill, or mussels are available for carnivores and frozen spirulina for herbivores. Tubifex worms and Mysis shrimp or other foods can also be found as freeze-dried cubes for carnivores.

Live Food: Options include live brine or ghost shrimp, feeder fish (for larger carnivorous fish), crickets, and worms.

Greens: Herbivores can be fed with greens like lettuce, cucumber, zucchini, and spinach. Clip the greens to the side of the tank or fasten them in place near the substrate, but remove or replace the uneaten vegetables within 24 hours. 

Different fishes need a variety of food. Therefore, if you have a variety of fish in your aquarium, use a combination of food—such as floating foods, slow-sinking foods, and rapidly sinking foods—to ensure they’re getting the nutrition they need.

There are a lot of alternatives available in homes that can be fed to fishes if you run out of fish food. These include green leafy vegetables like lettuce, spinach, cucumbers, sweet potatoes,  etc, cooked rice or pasta, hard-boiled egg yolks, fish fillets like cod and tilapia, or boiled peas. Also, one should also note the foods that are harmful to your fish’s life and always keep them at bay. 

Frequency and Quantity to Feed:

Adult fish can be fed once a day, around the same time, though you can feed them multiple times a day if you’re giving them a smaller amount each feeding. Young fish may need three or four feedings a day. Herbivores typically don’t have large stomachs to hold a lot of food, as in nature they would nibble on algae and plants throughout the day. They can be fed more frequently than carnivores, or given live greens that they can snack on throughout the day. As a rule of thumb, one should feed the fish only what they can eat in five minutes. If there is food left after that time (except for the fresh greens), it means fishes are being overfed. One exception is for fish that are nocturnal (night time) feeders, where you should put the food in the aquarium in the evening before turning off the lights, and let the fish eat overnight.

Fish owners are more likely to overfeed their fish than underfeed them, which increases the amount of waste in the tank. This is not only the waste left when the fish do not eat all the food but also the waste is excreted from the fish because they’re eating more than necessary. If you find that ammonia, nitrite, or nitrate levels are going up and the tank seems polluted, you’re probably overfeeding the fish.