The brightly colored small fish you see darting in and out of live rock are probably Damselfish.  They make up the family Pomacentridae.  Many damselfish stay relatively small but there are exceptions.

Damselfish make great additions too small to midsize aquariums because of their size and bright colors. They are extremely easy to care for and require little or no maintenance.  They are very good forages and eat very small  floating matter in the water columns.  It is best to add damselfish to a tank simultaneously because they can be very territorial once established.