Kelp is another name for several large, brown types of seaweed. Fish that eat kelp and other plants are called herbivores, in contrast to meat eaters, which are called carnivores. Some fish that eat kelp are true herbivores, while other fish are omnivores, meaning they eat plants and animals. Some fish will eat just about anything, including kelp.

Halfmoon fish are omnivores -- one of the types of fish that will eat almost anything, including kelp.The name halfmoon is derived from the shape of the fish's tail. In fact, the fish's botanical name, Medialuna califoriensis, means "halfmoon California" for the tail's shape and that the fish were first found in California. They are also sometimes called blue bass, blue perch or Catalina blue perch. Halfmoon are shaped similar to perch but are heavier and more full-bodied. Their coloring is darker blue on top and lighter blue on bottom. Halfmoon fish are found in shallow water near kelp.

Opaleye is another perch-type fish that lives in shallow reefs and kelp beds, and it is also an omnivore. It is dark, olive green with two whitish yellow spots below its dorsal fin and its tail is almost square. The opaleye lives in the Pacific Ocean, ranging from Oregon to Cabo San Lucas. In their first and second year of life, they inhabit tide pools and can breathe air.

Also known as the regal tang, palette tang, hippo fish and the blue surgeonfish, the Pacific blue tang is an omnivore. In the wild it primarily eats plankton, algae and seaweed or kelp, but in aquariums it will also eat bloodworms, shrimp and other seafood. The reefs of the Indo-Pacific, from East Africa to Japan, comprise its habitat. Some Pacific blue tang grow to be as large as 13 inches.

Kelp grows in huge ocean forests, mostly in the Pacific Ocean where the water is cold enough for it to grow. The forests grow in tiers, with a canopy on top and layers underneath. Most kelp falls into two types: giant kelp and bull kelp. Giant kelp is found mostly in southern California down to Baja, while bull kelp is found primarily in Northern California. Thousands of organisms live on the kelp and are eaten by fish, while other fish live in the kelp forests but don't eat the kelp. Larger fish sometimes hide from their predators in the kelp. Kelp is firmly anchored, but strong storms can break or uproot it and send the kelp crashing onto the shore.