I use to use distilled water until I found a water conditioner you add to basic tap water. I keep a 2 gallon jug full of tap water & the conditioner next to the tank for topping up. I also use this stuff called Melafix once a week. It's a medicament that helps with waterborne disease. There are other water treatments I add but the water conditioner and Melafix work wonders.
So far (knock on wood), we have a Danio pushing 10 years and a pleco pushing 8!
How big is your tank Claudia?