Fish can be omnivores, but some species are carnivores or herbivores. In general, a fish's diet is determined mostly by its habitat and what food is available. Most species have a preferred diet but will eat outside of that if necessary for survival.
The majority of fish are omnivores, meaning they eat both plants and animals. One example of an omnivore species of fish is the opaleye. This fish eats both the seaweed found in its habitat as well as the tiny creatures living on the seaweed. Other omnivore fish include African cichlids, angelfish, barbs and minnows.
Some people may think that fish are more likely to be herbivorous, but many fish live on algae and other marine matter that is usually comprised of small insects, micro-organisms and plankton. Strictly carnivorous fish are less common but include the popular betta fish, which prefers brine shrimp and blood worms, and the discus.
Aquarium fish are more likely to eat a limited diet because of the flake foods and pellets that are used by fish owners. These fish can easily get the nutrients they need from the food at hand. In the wild, the fish must eat whatever is available, which can also mean that they ingest chemicals and other toxins in the water as well.