Corals can be held out of water for several minutes or even a couple of hours at a time without being harmed. On wild reefs, Acroporids and Zoanthids are exposed at low tides on a daily basis, often for several hours. The corals can survive the harsh out-of-water experience, though I don’t recommend exposing a coral to air for more than a few minutes. Water keeps the temperature stable for the most part, while also providing oxygen and nutrients to the coral. Air has the tendency of being less stable in terms of temperature fluctuations and can cause the corals to stress out or even die. Also remember that not every coral is from the extremely shallow, often air exposed portions of the reef.