Corals require the following to survive in their environment.
- Sunlight: Corals need to grow in shallow water, where sunlight can reach them. Because corals depend on the zooxanthellae (algae) that grow inside of them and this algae needs sunlight to survive, corals need sunlight to survive too. Therefore, corals rarely develop deeper than 50 meters/165 feet.
- Clear Water: Corals need clear water to survive and do not thrive well when the water is opaque. Sediment and plankton can cloud the water, which decreases the amount of sunlight that reaches the zooxanthellae.
- Temperature: Reef-building corals require warm water conditions to survive. Different corals living in different regions can withstand different temperature fluctuations. However, corals generally live in water temperatures ranging from 20° Celsius to 32° Celsius, or from 68° Fahrenheit to 90° Fahrenheit.
- Clean Water: Corals are sensitive to pollution and sediments. Sediments can settle on coral, blocking out sunlight and smothering coral polyps. Pollution from sewage and fertilizers increases nutrient levels in the water, harming corals. When there are too many nutrients in the water, the ecological balance of the coral community is altered.
- Salt Water: Corals need salt water to survive and require a certain balance in the ratio of salt to water. This is why corals do not live in areas where rivers drain fresh water into them.