1) Sunlight: Corals need to grow in shallow water where sunlight can reach them. Corals depend on the zooxanthellae (algae) that grow inside of them for oxygen and other things, and since this algae needs sunlight to survive, corals also need sunlight to survive.

2) Clear Water: Corals need clear water that lets sunlight through to survive. They don't grow well when the water is opaque as sunlight cannot reach them. Sediment and plankton can cloud water, which decreases the amount of sunlight that reaches the zooxanthellae.

3) Warm Water Temperature: Reef-building corals require warm water conditions to survive. Different corals living in different regions can withstand various temperature fluctuations.

4) Clean Water: Corals are sensitive to pollution and sediments. Sediments can settle on coral, blocking out sunlight. Pollution from sewage and fertilizers increase nutrient levels in the water, harming corals. When there are too many nutrients in the water, the ecological balance of the coral community is affected.

5) Saltwater: Corals need saltwater to survive and require a certain balance in the ratio of salt to water. This is why corals don't live in areas where rivers drain fresh water into the ocean.