Is it safe to use distilled water in a fish tank?

The water where I live is very hard, and i was sick of cleaning the white scale off of my 20 gallon tank. I've filled my tank with distilled water because I know it will not have any of the minerals at all. I've been looking around the internet to see if it is safe to use a purely distilled water tank and I am under the impression that it is not a good idea. There are currently no fish in the tank, and I have a long bubble stone installed. Other than that, I have access to aquarium salts, stress coats, and pH indicators. I'm at a loss of what to do, and I'm very hesitant to use tap water again. Thank you for your time and concern.