I was wondering why you didn't mention the distilled water Ed. I would love to read any online links you might have. I have always thought distilled water as one of the ultimate "no, no's" in frogkeeping, if I can avoid these damn water stains, let me know!JVK
I don't believe the information has been published for free on the web.
If we look at the history of the issue, it started with several articles discussing ion loss to severely hypotonic solutions primarily by tiger salamander larvae which were summed up in Amphibian Medicine and Captive Husbandry (2001, Kreiger Press) with the recommendation to not use distilled or RO water with animals as it causes the loss of some ions into the solution (calcium) through the osmoregulation. The more recent data came out in as series of experiments in Ecological and Enviromental Physiology of Amphibians (2009, Oxford Press). In those the studies were run for a longer period and it demonstrated that yes those ions were lost to the solution but the amphibians also were able to actively reabsorb those ions back from solution. The loss is basically energetic unless the amphibian has something compromising the ability to osmoregulate such as septecimias or chytridmycosis. So as long as the amphibian is healthy it is in reality fine to use and you can get away from those calcium stains.
If we step back and look at it clinically, technically tap water or filtered water or even pond or stream water is also significantly hypoosmotic in potential to the amphibians as the osmolarity of amphibian plasma is much higher than that of those water sources. The impact is the same, the amphibian loses ions to the water and reabsorbs them. Somewhere on forum I think have some of the data on amphibian ringers compared to pedialyte and why pedialyte shouldn't be used in cases of suspected hypocalcemia.
In animals with compromised abilities to osmoregulate, amphibian ringers is suggested for a number of reason as it reduces stress on the system of the amphibian and in cases of extreme water retention, under the care of a vet, hyperosmotic amphibians ringers are suggested.
In cases like the above, where the water is coming into contact with a variety of materials the water is als still going to be hypotonic with respect to the amphibian but is going to have picked up stuff from the enclosure which is going to go into solution.
Does that help? Sorry it isn't on the web for free.
Ed