Most buyers go with Salt water fish, other than freshwater fish, mainly becuase salt water fish are generally more colorful
Reef Fish
Reef fish travel in schools and get along with other reef fish and generally are more colorful, and can live in smaller tanks.
Predator Fish
Predator Fish are usually bigger and require larger tanks and do NOT get along with other fish, sometimes not even with their own kind, they are also less colorful, most are just plain gray.
Other Tank Life
Other life in your tank is required in order to have healthy fish, and a beautifal aquarium.
Reefs
Reefs are escential for reef fish they feel better protected and gives them a sense of privacy, and depending on the fish you have you also might need anemone, though may not be predators they need it for a home and protection from rival fish, when they lay eggs.
Crustation
Crustation are needed to keep the bacteria at bay and do a great job at sanitation, they also add a sense of class to your tank.