Water for filling a newly purchased aquarium is available from a variety of sources. Using tap water, which has been treated with chlorine for human use, can be deadly to aquatic pets. The chlorine needs to be removed by using dechlorinator. This product instantly removes chlorine and chloramines from water.
You can use dechlorinated tap water since most purchased freshwater fish are raised in a hatchery with standard aquarium water conditions. All the water added to an aquarium must be dechlorinated, even the water put it the aquarium to replenish water lost to evaporation. Some hobbyists collect rainwater for aquariums. Contaminants from factories, smog, and other pollutants in the water must be treated before using it. (This could end up being a costly process for aquarium setup.)
Your fish do not need to be placed in water from a pond or river, as fish easily adjust to tap water that has been dechlorinated.