From FrugalReef

Doing a water change is a technique for reducing accumulated toxins, minerals, nitrate etc from the water. The common rule of thumb is to change 10-20% of the water twice a month, but this rule is a complete generalization at best.

A few things to keep in mind, however:

• Don't change too much of your water at once, quickly changing water parameters can be as deadly to your reef inhabitants as the toxins you're trying to fight.
• Heat the water before you put it into the aquarium, or put it in slowly enough that the aquarium temperature doesn't fall.