If they're fresh water, they get tap water + seachem prime. Never had any issues with any fish.
Only reason for RO/DI is for some salt applications, or if you have something super sensitive that needs very specific conditions.
RO/DI water is not good to go strait out of the filter. It's like a blank canvas. You need to add things to it to make it right. It'll allow you to change PH, hardness, etc... very easily, where as using my tap water for example, I'm stuck with whatever it is since it's got so much buffer in it. If I try to change the PH, it'll just go right back with my tap water.
