A Reef Tank is a marine (salt water) aquarium which contains plants and animals most often found in reefs. These tanks often contain more plants and invertebrates and fewer fish, with "live rock" and "live sand" as an important component.
The invertebrates typically include anemones, corals, and crustaceans.
Both anemones and corals contain symbiotic algae which photosynthesize ambient light. The algae produces excess sugars which enables the larger creatures to survive even when food is scarce. Because of this, use of intense light is important in a Reef Tank.
Anemones are soft creatures with only a single layer of skin and muscle. Their "bulk" typically consists of water which they pump inside their skin sacks to give themselves some heft. Their tentacles are often stinging and adhesive, and they are used to carry bits of food to the centrally located mouth. When "deflated", they can shrink down to almost nothing. They can and do eat solid food, but need this only on occasion.
Corals come in a variety of types. Soft corals are mostly "flesh" and can be shaped like mushrooms, fans, or trees. Some hard corals appear to be completely hard (with virtually no observable soft parts); they contain living polyps, however small, which they can use for both photosynthesis and eating bits of food which pass by. Some corals consist of a hard, bony part in which the creature lives, but they project their polyps in order to feed. These corals show a surprising range of shapes as their polyps change during the day.
Crabs, scallops, and clams are also part of the Reef Tank mix. They key issue with most of these creatures is selecting a specimen which will not grow so large as to endanger other members of the tank.
Live rock and sand provide homes for a variety of bacteria and algae. The algae help convert light into food energy for the herbivores in the tank. The varieties of bacteria convert the chemicals which the animals produce (most notably ammonia) into less noxious forms which can either be metabolized by algae as nutrients or can be "skimmed" out of the system.
The reason for minimizing the number of fish relates to the desire to make the Reef Tank as much of a "closed system" as possible. By balancing the animals and plants (and by minimizing the carnivores), one can create a mini ecosystem which becomes close to self sustaining. Many of the animal end products can be re-processed by the algae and bacteria which live in the live rock and sand. The system is somewhat self adjusting, with the addition of more fish creating more ammonia which allows more bacteria to grow, which produces more nitrogen, which helps the algae grow. Reef Tanks must host this cycle (the Nitrogen Cycle) properly in order that the tank does not "crash".
Some animals are great in Reef Tanks, because they are herbivores which will not bother the other inhabitants of the tank. Other animals are either too aggressive, or have a diet which would destroy the tanks contents rapidly. Thus, as much as I might want to have a lobster or octapus, these belong in separate tanks.
The Reef Tank physically consists of the tank (typically between 50 and 300 gallons); a lighting system (typically with timers); a pump (to move water between the tank and the filtering system and back into the tank); a mechanical filter (to trap bits of sand and algae before they enter the pump and damage it); and a "skimmer" (to isolate noxious biochemicals for removal). Every week small water changes (about 5%) are performed every week and water chemistry is tested. Costs range from $20 to $30 per gallon after the expense of both equipment and specimens is taken into account. Timers can be used to create light levels corresponding to night sky, pre-dawn, dawn, early daylight, and high noon.
I hope this has helped you get a feeling for some of the issues which will be present in the Reef Tank photographs which are on other pages of this site.
Feedback about this writeup would be appreciated.