Do NOT use water that is labeled as "distilled" or "drinking water". Distilled water (or Reverse Osmosis water) has virtually no ions in it, so it causes more work for the amphibian's kidneys. It may be used to replace water lost to evaporation, but an animal should not be kept in pure distilled or RO water. Bottled "drinking water" is usually just filtered tap water and probably no better than your own tap water.
The Wright & Whitaker Amphibian Medicine and Captive Husbandry book says:
"Amphibians rely upon the water in which they live to provide them with many of the constituents needed to carry out metabolic processes... Distilled and reverse osmosis water contain none of the elements required by amphibians and should not be used without modification."