Although they vary dramatically from one another in appearance, these animals are all members of the phylum Cnidaria and are thus closely related. Until recently, corals were considered by most as nearly impossible to keep and, indeed, they still present problems. Water quality is exceedingly important, as is the wavelength and intensity of lighting. Many coral species obtain a good deal of their food via the action of symbiotic algae, which live within them. Without proper lighting the algae perish, and no amount of additional food provided thereafter can keep the coral alive. Most corals feed upon plankton-sized food items, and although commercially prepared diets are now available, many are still difficult to maintain. One exception is the tooth coral. Now routinely available in the trade, this species eats large food items, such as pieces of shrimp, and is thus fairly easy to feed.

Jellyfish (see color insert for photo) are increasingly exhibited in public aquariums but are, as yet, too delicate for most home aquarists to maintain. Indeed, very few are offered for sale. One possible exception is the upside-down jellyfish (Cassiopeia andromeda). In contrast to nearly all of its relatives, this jellyfish spends its life resting on the substrate with its tentacles trailing in the water currents above. Much of its food is produced by a symbiotic algae, so intense lighting is absolutely necessary. This jellyfish will also feed upon newly hatched brine shrimp, but it cannot compete with faster-moving invertebrates or fish and so must be housed accordingly. It is, at present, the only species regularly offered for sale.

Sea anemones are among the heartiest of the cnidarians, although even they will perish quickly if kept in suboptimal water quality. Most also require a fairly steady current of water flowing over them at all times. A very commonly available species is the Caribbean anemone (Condylactis gigantea). Varying in color from white to brown or pink, this species is quite hearty, but unfortunately it is rarely adopted as a home by clownfish, which often find shelter in the appendages of anemone. More attractive to these popular fish is the purple-based anemone (Heteractis magnifica). This is a very active species that quite frequently travels about the aquarium. All anemones will thrive on infrequent (once or twice a week) meals of shrimp, clam, fish, and similar foods.

All species of anemone, coral, and jellyfish possess stinging cells that are capable of causing painful rashes, and none should be handled with bare hands.