In order to have a nice-looking saltwater aquarium, you have to keep corals happy.
Corals are Animals
Reef keepers aim to keep some of the most exotic and unusual animals that come from the farthest corners of the Earth inside their little glass boxes. This is no small endeavor. These animals usually have very specific requirements, and sometimes two animals that look very much alike have completely different requirements.
If you want to do your corals justice and take good care of them, you need to be well educated about their care and natural history. So do some research and read up.