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 these algae needs sunlight to survive, corals also need sunlight to survive. Corals rarely develop in water deeper than 165 feet (50 meters).
Clear water: Corals need clear water that lets sunlight through.
Warm water temperature: Corals generally live in water temperatures of 68–90° F or 20–32° C. They have tolerance to a very narrow temperature range.
Clean water: Corals are sensitive to pollution and sediments. Sediment can create cloudy water and be deposited on corals, blocking out the sun and harming the polyps. Waste water discharged into the ocean near the reef can contain too many nutrients that cause seaweeds to overgrow the reef.
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 (“estuaries”).