In most cases tap water is the easiest and friendly choice for organic growers, However there's some few factors to look into for best safety approach which is pH range, Total dissolved solid or TDS ppm levels and lastly, to get rid of the extra chlorine.
The optimal pH level of tap water could be anywhere around 6.2-7.5 with total dissolved solid about 100-250 ppm and since most household water usually goes through the water purification system like carbon or resin filter before it enters the tank, there should be no problem at this point.
If your pH level and TDS ppm is on point there's only one more thing to do before you can use tap water safely indoors, to remove the extra chlorine by using an aquarium oxygen pump. Leave it in the water tank for at least overnight with the lid open to allow the chlorine to evaporate. We would recommend keeping the pump on at all times.
The same method can be applied to rainwater and groundwater.
If none of the above options could be used, Reverse Osmosis is a way to go. RO water is very clean and usually has the ideal pH levels but because most nutrients will be filtered out almost completely. Therefore, the water has insufficient properties to exchange ions so to be able to use RO water, calcium and magnesium or often known as cal-mag must be added before use.
Notes
pH down = Apple Cider pH Up = Baking Soda Cal-Mag alternative for RO water = Coconut Juice (2 tsp : 5 liters of water)