For a while I used a 10-gallon tank with water (distilled) at the bottom and an aquarium heater for my Ampullaria and Bicalcarata. The plants were perched atop upside-down flower pots; they're quite young so this setup worked. Recently, however, I acquired a great many more adelae (which also inhabit the 10 gallon) and this has forced me to rearrange my current setup.
As a result I broke out my old hexagonal aquarium and have, again, placed my lowlanders atop upside-down flower pots within it. The tank is quite large so they should have ample growing room. But now I'm left with trying to raise both the humidity and temperature, and I want to again try the technique of filling the base with water and heating it with an aquarium heater. In order to save money, though I thought I'd ask (but I'm quite sure I know the answer already) whether filling the base with non-distilled water (straight from the tap) and heating THAT was safe for the Nepenthes. I would never actually water the plants themselves with it. I'm hoping to limit the # of gallons I have to buy each week of distilled water.
I haven't tested my tap water's PPM, but I imagine it's too high to use to directly water the plants.