This has become an interesting discussion.
The last thing I want to do is end up with fluctuating pH and stressing out my betta. So at least for now I'm sticking with the bottled drinking water, which I've used for over a year, and just going to try a little mixing of well water and drinking water in jars just to see what parameters I end up with. I've read that distilled water isn't good because it's totally lacking in minerals. So is drinking water okay for long term use? Or is my betta not able to get what he needs from that water? That was one reason I thought of mixing well water and drinking water. The pH of the drinking water, when taken from the tank, was 7.0 or 7.2, so I probably don't really want it much higher than that, right? I was wondering if mixing with well water increased it somewhat, whether there was something safe and natural that I could add to offset that, but if I don't want darkened water, it seems I may be stuck with the drinking water.
I'd be interested in hearing how the pillows work for you, Bugsieb63. I saw them in the Drs Foster & SMith catalog and wondered if they were any good. I considered switching to spring water too, but sounds like, from your experience, that this isn't the answer either.