What Do Fish Eat? Get To Know Their Diets

feeding swarm tetra aquarium fish eating flake food

Scientists have identified over 33,000 different fish species, so it’s no surprise that there is a lot of variation in their diets. Generally, fish can be grouped into broad classes based on the source of their nutrition.

So, what do fish eat? Fish eat a wide variety of things! They can be broadly classified as herbivores, carnivores, or omnivores, which describes the source of the foods they eat (plants, other animals, or both). Different species thrive on varying amounts of proteins, fats, and carbohydrates (macronutrients) and have unique vitamin and mineral (micronutrients) needs.

Fish that are herbivores, carnivores, or omnivores all have different nutritional needs and food preferences. In captivity, this translates into multiple kinds of fish foods like pellets, flakes, and freeze-dried foods with variations in amounts of macronutrients and micronutrients.

In this article, we will discuss what herbivorous, carnivorous, and omnivorous fish typically need and the different food products offered at pet stores.


By far the smallest group, herbivores eat foods made entirely of plant and algae materials and are essential to maintaining healthy reef ecosystems.

In the wild, reef herbivores mainly eat red, green, and brown algae and do not require any meat or other animal as part of their diet. Surgeonfish and tangs are part of this group.

Herbivorous fish do not have a true stomach but instead have a long gut and intestine that allows for extensive digestion required for plant materials. They use mechanical digestion or enzymatic digestion to break down the food as it passes through.

Mechanical digestion is a similar process to chewing, as it uses grinding to break the algae’s cell walls. Enzymatic digestion involves the production of enzymes by microbes in the gut that can breakdown the tough polysaccharides of the algae into molecules that the fish can absorb.


Carnivorous rely solely on meat or other animals for food and typically eat insects, crustaceans, or other fish in the wild.

Common foods are crickets, snails, worms, brine shrimp, and fish that are smaller than themselves. Along with the obvious carnivores like piranhas, other group members are tetras, betta fish, and oscars. 

Carnivorous fish have digestive systems that are great at breaking down protein but cannot efficiently digest carbohydrates or plant material. Unlike herbivorous fish, carnivores have a stomach that aids in digestion and a relatively short intestine.

They utilize stomach acids and pancreatic enzymes to break down the stomach’s food into more easily absorbed particles.


The omnivorous group of fish is the largest of the three. They eat plant and algae material as well as food sourced from other animals. Their digestive tracts have evolved to handle both carbohydrates and proteins, with some variation in species’ exact process.

In the wild, omnivorous fish typically eat algae and insects, other fish, or crustaceans. Some well-known fish that fall into this group are angelfish, goldfish, and parrotfish.

Nutrients and Their Roles

As you can see, each group needs its specified food type to obtain the most nutrients possible from digestion. But what exactly are those nutrients, and what do they do? 


Proteins are made up of linked amino acids, which are important for bodily processes like building muscle and supporting the immune system. There are quite a few amino acids that exist, but 20 are needed to maintain optimal health.

10 can be created inside the body or synthesized, but 10 cannot and need to be obtained through diet (essential amino acids). It is important to make sure that the feeds you use for your fish provide adequate amounts of all 10 essential amino acids, as the amounts vary among protein sources.

Protein needs vary depending on the species and age of the fish. Carnivores and omnivores typically have higher protein requirements than herbivores. Some carnivores require that up to 45% of their diet come from protein.

For small species and growing fish, the recommended amount is higher than for adults of the same species. 


Although carbohydrates are not essential to fish’s health, they are the least expensive commercial fish food component, so they are used to reduce cost. Carnivorous fish foods typically contain less than 20% carbohydrates, while herbivorous or omnivorous foods contain around 25-45%.

However, only a fraction of the energy stored in carbohydrates is extracted and utilized by the fish, with carnivorous species being the least able to digest them.


Fats do not make up a large part of any fish diet as too much can cause fatty liver disease. However, they do play an important role in the overall health of fish.

Fats are necessary to absorb fat-soluble vitamins like A, D, E, and K, and omega-3 fatty acids are required for optimal health in many species. Usually, fats make up anywhere from 7-15% of a fish’s diet. 


Fiber is composed of the plant materials lignin and cellulose, which are indigestible carbohydrates. Since carnivores’ digestive tracts are not suited to carbohydrates, they should not get more than 4% of their diet from fiber (on top of other carbohydrates).

Herbivores can stand to have fiber make up between 5-10% of their overall diet.

Commercial fish foods are formulated using byproducts from the agricultural industry. Bone meals, blood meals, feather meals, and even fish meals are animal sources of protein.

Plant meals, grains, and corn and wheat gluten are vegetarian sources of proteins. Fats come from plant oils and animal fats. Cereal grains and byproducts of milling make up a large part of the carbohydrates.


Along with macronutrients, fish also need vitamins and minerals. The amount needed and the importance of each vitamin depends on the species of fish. Here are the vitamins that fish owners should monitor:

  • Vitamin B1, B6, and pantothenic acid, riboflavin (B2), biotin, and niacin: Important for metabolizing food and carrying out the breakdown of proteins, lipids, and carbohydrates.
  • Choline: Component of cell membranes and supports nervous system function.
  • Folic acid: Assists in the production of hemoglobin and the synthesis and metabolism of different amino acids.
  • Vitamin B12: Assists in the production of red blood cells and maintenance of nerve tissue.
  • Inositol: Component of the brain, heart, and skeletal tissues.
  • Vitamin C: Plays a role in collagen production.
  • Vitamin A: Required for vision and maintenance of epithelial tissues throughout the body.
  • Vitamin D: Helps to deliver calcium to bones.
  • Vitamin E: Acts as an antioxidant to protect cellular membranes and other compounds.
  • Vitamin K: Required for blood clotting.

More in-depth info about the roles and mechanisms of each vitamin listed can be found here.


Like vitamins, the amount of different minerals needed for optimum health varies by each species of fish. Here are some important macrominerals:

  • Calcium: Component of the skeletal system and essential for blood clotting.
  • Phosphorus: Component of the skeletal system and essential for the balance of body pH.
  • Magnesium: Component of the skeletal system and plays a role in macronutrient metabolism.
  • Sodium, potassium, and chlorine: Essential to maintaining body fluid pH and controlling fluid exchange with the surrounding water.
  • Sulfur: Component of multiple amino acids and insulin.

There are also microminerals, including iron, zinc, manganese, copper, cobalt, iodine, selenium, and chromium. They are the main components of various enzyme and hormone systems and are also required for optimal health.

More info about the macrominerals and microminerals can be found here

Overall, it is important to ensure that you meet your fish’s nutritional proper ratio of macronutrients and the proper amounts of vitamins and minerals.

These needs vary greatly among different species, so it is necessary to do some research before purchasing fish food.

Commercial Fish Foods

Commercial fish foods add simplicity to many fish owners’ routine because they offer the ideal macronutrient ratios and vitamin and mineral contents for different fish. These foods can be found in many forms, so let’s take a minute to go over your options.


Pellets are ideal for larger fish and have carnivorous and herbivorous options. They come in both floating and sinking varieties that you can choose depending on your fish’s preference. There are pellets available for many individual species that are tailored to their nutritional needs.


Many species and sizes of fish readily eat flakes. They come in varieties for saltwater fish as well as for freshwater fish that live in groups.

The flakes sit on the surface of the water for a brief period before slowly sinking to the bottom of the tank, so they are best suited to fish that inhabit the upper and middle levels of the water column.

It is best to buy only a month’s supply at a time, as flakes lose their nutritional value relatively quickly once opened.

Freeze Dried or Frozen

The benefit of freeze-dried or frozen foods is that they have a longer shelf life and are safer than live feeds but offer fewer nutrients. These foods are beneficial for providing variety in the fishes’ diet and catering to “specialist” feeders that will only eat particular foods.


Live foods are as close to “in the wild” feeding as you can get and have high nutrient content for your fish. However, they are more expensive than commercial foods and have the potential to bring diseases or bacteria into your tank.

When to Feed and How Much

Now that we’ve covered what your fish needs in their diet and the kinds of commercial fish foods available, you may be wondering how much or how often to feed your fish.

Like exact nutritional requirements, this answer varies greatly depending on the fish species, their age, and how many are in the tank. 

Some species eat at particular times in the wild, like dawn and dusk, while others are foragers that eat little amounts throughout the day. Also, younger fish that are growing may need to eat more frequently than older fish. 

A general guideline is only to feed what is eaten within a few minutes. Giving too much food can result in uneaten particles dispersing throughout the tank, which can dirty the water and clog up the filter, among other problems.


There are many commercial fish foods available to pet owners, and the number of choices can be overwhelming.

It is best to determine each fish’s individual nutritional needs in your tank, then take into account the preferred feeding style and frequency of each fish and whether they occupy the top, middle, or bottom of the water column in their habitat. 

Once you have gathered this information, you can narrow down the available foods to which ones meet nutritional requirements and match the feeding style of your fish.

It may seem like a lot of work to select the appropriate fish food, but your aquatic companions will thank you.

John Kilmerstone

I love keeping pet fish and receive a lot of joy and peace from watching these colorful creatures. Please visit this website and explore the wonderful world of pet fish. Discover how to care for and look after pet fish and amplify your satisfaction.

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