You can also acidify the water down to a reasonable pH, though even phosphoric acid is somewhat dangerous and the best choices (nitric or sulfuric) aren't something I want in my apartment.

Amateur hydroponic growers often use reverse osmosis water, somewhat wasteful but cheaper than distilled. You can get RO systems designed for aquarium use with no pressure tank at the output and just let them slowly fill a barrel or tote, over hours or days.

Don't know why I got this reply? Green onion bottoms that normally would go into the compost aren't expensive, and the tap water here is very soft.

What about tap water from countries where the water is soft, like Scotland, Ireland or Norway? Or is that still too "hard" for plants?