Sammy and Beth are right!! Also you can see where the fish you already have like to swim, if they always hoover in one spot, or do they like the bottom or the middle. They kinda get their space in the tank, if you add to many fish suddenly each fish doesn't get it's own space. I definitley think the hiding spaces are important to. Most fish like to hide somewhere at night to sleep, just like us we go to our beds at night, the fish go to theirs. So if you have 5 fish it is a good idea to have 5 "beds." Each one of my fish has their own little spot to sleep, some in the barnacle, another in a rock hole, and one in a coral hole, ect... but they each have their own personal space.