Simply put, a "reef tank" is the closest thing to the reefs in the ocean that you can put in a glass box. Due to the size difference, the things you put in your reef tank have to get along with each other. In the ocean, a species being predated by another can move or migrate to a safer area. In a closed system like your 55g tank, the opportunity to flee is greatly limited. For this reason, the critters that you put in your reef tank have to be able to cohabitate peacefully.mari.