Top Off Water
Water will evaporate out of all aquariums. The rate of evaporation will depend on whether or not your aquarium has a full cover, the type of filtration used, and the temperature of the water. Typically, for most aquariums, you will have to top off the aquarium water weekly, if not more often.
When water evaporates it leaves behind minerals, and nitrate. Topping off the water will often introduce more minerals, and in some cases, additional nitrate. Overtime minerals and nitrate can build up beyond recommended levels, if water changes are not conducted on a regular basis. In aquariums with heavy plant growth, adding additional minerals may be necessary, as the plants can remove them from solution as they grow.
Water changes are primarily performed to lower the nitrate level in the aquarium. Regular scheduled water changes are often essential to maintaining a healthy aquarium ecosystem. How often you need to do a water change and how much will be determined by how fast nitrate accumulates in the aquarium. You should look at nitrate as being smog in the aquarium. The higher the concentration of nitrate, the greater the pollution.
The solution to pollution (nitrate) is dilution. A typical water change in the author's aquariums. Water is drained down to the point where the fish just have enough water to stay wet. The sand bed is never disturbed when removing the water or filling the aquarium up. A dechlorinator is added to the aquarium before filling the aquarium up with water that is within a few degrees of the tank water temperature. Water chemistry is immediately adjusted after adding the new water.
In a freshwater aquarium you should try keep the nitrate level as close to 0 ppm as possible. For most freshwater aquarium systems, nitrate will accumulate over time and should not be allowed to exceed 100 ppm for more than two weeks.