The diversity of coral reefs is second only to the diversity found in

Tropical Rain-forests

Explanation:

Tropical Rain-forests specifically those in Central Africa, Latin America and South-East Asia are considered the most diverse areas in the world with over half of animal species in the world as well as countless fauna calling these areas home even these forests cover less than 7% of the Earth's land mass.

For this reason Tropical Rain-forests are usually called the Cradle of life/biodiversity and surpass any other environment when it comes to biodiversity. Even the second which are Coral Reefs are known as Tropical Rain-forests of the sea which is a nod to just how diverse these forests are.