The outer reef (or continental shelf) is the part of the reef that looks like an underwater "cliff", in which the reef deepens sharply, even completely vertically. Most corals live there, because the water has more currents, thus better nutrition and zooplankton circulation. As it seems, these areas are an ideal place for many fish, as they host many different corals that serve as a home, hiding place, but also food for them. Naturally, the competition for these positions is huge! Corals can only live in tropical seas, where temperatures range between 20-30°C. This temperature is ideal for the solidification of limestone, where it forms the basis of coral (limestone skeleton).