I've been told by amphib breeders never to use distilled water. It is pure water with no salts or electrolytes, but amphibians have salts/electrolytes in their cells. Because of osmosis, these salts etc need to equal out and because amphibs breath through their skin, water will be "sucked" into the animal to even out the concentrations and proceed to burst its cells.

Tap water isn't perfect either as it may contain chlorine and other chemicals which are dangerous for the amphibs too, but you can treat this water with dechlorinator to make it safe (you can get stuff designed for amphibians rather than using the stuff designed for fish).