Sea stars have distinct upper and lower portions. On the underside of each sea star is the mouth along with a distinct cleft through which food is moved to the mouth. This cleft and mouth is referred to as the oral disk. Some sea stars do not have a separate opening for removing waste and as such, their "mouths" act as both the site for the import of food and the export of waste materials.

Most sea stars are rather specialized feeders, feeding on particular food items but many can become omnivorous general feeders. The most desirable varieties to have in a reef aquarium are either the detrivores or the algae eaters. These include members of the genera Linkia and Fromia. These stars seldom bother corals, but they need to be kept in a well-established aquarium that contains lots of microfauna as well as microalgae upon which they can feed. Sea stars can be omnivorous or specialized feeders so before any are added to a reef aquarium, their feeding requirements should be researched.

There are several predatory starfish that should not be kept. The Crown-of-Thorns, infamous for consuming large portions of the Great Barrier Reef in recent years, is probably the best-known example. However, a more problematic predatory one is the small sea star of the genus Asterina, introduced to many reef aquariums in recent years. This tiny sea star is usually no bigger than 1 inch across and seemingly has two color varieties. The white or gray variety is usually a harmless algae eater that consumes microalgae from the glass and live rock. However, there is another similar looking sea star that is mottled red, pink or green that feeds aggressively on stony, and in some instances, soft corals. This pest has become a frequent hitchhiker on Indo-Pacific live rock and corals. They can grow rapidly to a population that can decimate stony coral colonies in an aquarium. All new coral or live rock additions should be carefully examined for these sea stars and removed immediately.