If you are only topping off, distilled water or RO is a great choice. When water evaporates from your aquarium, it leaves all the chemicals behind. You are basically running a very slow still that gets rid of pure water. Replacing it with pure water is a great idea. The cost can catch up with you because the fish are adding impurities all the time. Because of the impurities you must do partial water changes even if you use pure water fo makeup. The purified water is going to codt you and then you will drain some of it out to replace it when you do the mandatory water changes. It is almost a better idea to limit the amount of water between changes so that you only replace the contaminated water that a partial water change is intended to replace. The down side is how quickly the tank level can drop if you don't replace losses. I use RO water for the simple reason that it adds no chemicals but replaces the water with almost pure water. The end result is no build up but even home made RO water has a cost in terms of waste water and filter maintenance.