I used to use aged tap water, but I live in central Arizona, where the water comes out of the tap in little pellets of calcium. It's so hard, we drive our cars across the lakes...
I did get the deposits on my cage glass, but my main concern was giving my snakes kidney stones. I also keep a tropical fish aquarium, and one day I decided to seek a blend of water that would be acceptably soft, but still provide some minerals. I used my test kits from the aquarium to determine hardness in my water mixes. I settled on half-and-half tap water and distilled water. This mix has a hardness of only three degrees GH, appropriate for Amazonian tropical fish. I started using it with my boa constrictors, and now use if for all my herps.
Pure distilled water has beem blamed for bone density loss in small animals and even people, thus the motivation for not using pure distilled or RO. After several years of using my blend of water, I have had no ill effects, and even my jungle tropicals are doing well.
I also keep all my herps under full-spectrum lighting, which may or may not contribute to their vitality. The favorite theory is that snakes arose from a burrowing ancestor, which may explain why they don't seem to require full-spectrum light to survive--their burrowing ancestor adapted to survive in a lightless environment before they resurfaced and radiated. I see it as a quality of life thing. The snakes are active and show superb feeding and breeding response. It's my subjective impression that they do better now than in the old days when I did not use full-spectrum light. Maybe I just feel better about it and imagine that they are happier.
