Top Off Water
Water will evaporate out of all aquariums. The evaporation rate will depend on whether or not your aquarium has a full cover, the type of filtration used, and the water temperature. 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. Over time minerals and nitrate can build up beyond recommended levels. In aquariums with heavy plant growth, adding additional minerals may be necessary.
Fish food also adds minerals and nutrients that aquatic plants can use. Over time, the nutrient concentration can promote excessive algae growth.