You need something to build up beneficial bacteria in a tank and maintain it. Corals will do the job after the tank is cycled. There are many people that will only keep a Few shrimp and maybe one fish in their reef tanks. Just make sure you have the proper levels before adding coral to include
PH
ALK
CALCIUM
MAGNESIUM
STRONTIUM
Ammonia
NITRITE
NITRATE
IODIDE