Which environmental factor determines why coral reef populations are found only in shallow waters?

Corals are the important part of marine ecosystems. They are found in both deep water and shallow water. However, coral reef are found in shallow water. The environment factor on which coral reef is depends is the presence of sunlight. In shallow water where sunlight can reach because corals depend on algae zooxanthellae. These algae grow inside them and provide oxygen for them through photosynthesis. Besides coral need clear warm water to grow which is present in the shallow region of an ocean.