Fish diets vary by species. There are some carnivorous (meat eating) fish, some herbivorous (plant eating), and many omnivorous (both meat and plant eating) fish. For example, the salmon is known as a predatory carnivorous fish, while the saltwater angelfish is herbivorous and the opaleye is omnivorous. The opaleye lives in the Pacific Ocean and feeds on both seaweed and the tiny creatures that live in and on the seaweed.
The diet of specific fish within each category of herbivorous, carnivorous or omnivorous depends on what is available in their habitat. For example, herbivorous tropical reef fish will focus on eating plant matter that is available to them in a reef setting, including a variety of different kinds of algae. These fish provide an important ecological service in consuming these algae by preventing an overabundance of this microscopic plant matter, which can have a negative impact on coral growth. Similarly, predatory carnivores can have a negative ecological impact if they are introduced into non-native areas by displacing the top predator, causing a disruption in the food chain. One such example of a disruptive carnivorous fish is the snakehead fish, which has become an ecological nuisance in Maryland and Florida.