I think I found my solution...

I took some "snow water" and tested it. After it melted, of course, LOL. It's GH is a profound Zero.

And I tasted it, being the nutcase I am, I wanted to make sure there were no detectable impurities, anyway. Tasted like bottled water, to me.

So it looks like treated tap water and snow water is going to win...

Until another month or so, when we finally start getting unfrozen precipitation again.

:)

I think those pillows may be useful with rainforest fish which may be a little calcium sensitive. Maybe they can be tricked into spawning.

You are correct in being concerned with just topping off the tank with tap water.

Is your rain clean enough that after 20-30 minutes of rain (to clear the air and roof of dust, etc.) that you could put a fish bucket under a rainspout? Sometimes, if the water is a little odd smelling, a filter with activated carbon can be put in it. At a certain point the cost of the activated carbon makes it easier to buy demineralized water at the store. That is still way too expensive to do much.

A few years ago, when my Dad was about 87, his water softener (or maybe water heater, but this is a favorite story) gave out. The guys who came to replace it asked him if he wanted the one with the 20 year warranty or the 30 year warranty. He looked at them and at his reflection in a window pane. Then he just began chuckling. ;)