The stony corals in our reef aquarium are representatives of the sorts of corals that build reefs. Techniques for keeping such corals alive in captivity have only recently been developed. Several decades ago, there were only a handful of people in the world who had successfully kept these corals, and methods only started to become widely understood in the United States in the late 1980's or early 90's. Even now, reef tank methodology is evolving rapidly. Most of the major breakthroughs in reef aquarium husbandry have been worked out by amateur aquarists, and most large public aquaria only began setting up reef tanks housing live corals in the late 1990's.
Most people think of corals as just pretty white rocks.
Indeed, calling a coral skeleton a coral is like calling a human skeleton a human. Though most corals and anemones can capture small animals or other food with their stinging tentacles, many species have symbiotic algae called zooxanthellae living within their cells. These algae photosynthesize, using sunlight energy to manufacture energy-rich food molecules such as sugars. Some of these products of photosynthesis leak out of the algae cells and into the coral or anemone cells, thus feeding the animal hosting the algae. The stony corals that build coral reefs are particularly dependent upon their resident algae, which is one of the main reasons why coral reefs only develop in shallow, clear water with good sunlight penetration.