@RosaB
I've started using rain water. I'm going to rig a rain barrel to catch the run off from my roof and install a sill cock at the bottom of it.
So far just a few carefully placed coolers right b4 rain storms has yielded me about 15 gals per storm even though it's winter time here in New England (if we even want to call this a winter).
For me the ppm has come out around 5.
Please correct me if I'm wrong but I've read you can't use a pH meter with those few ions but I once I add nutrients I'm coming out around 4.5 pH range.
That's easy to correct with pH up (4 mL /gal for me fixed it)
round here rain barrels ( not old chemical barrels food barrels like pickles or olives) are quite common and many towns are happy to provide them (just like composters) at a good discount . I can also blend that with my tap water so I end up above say 50 ppm but (my choice ask MOG for a better range) below 100 ppm.
Also like others have said you can buy water even by the gallon. Here market basket sells gal that are about 1 dollar and have a ppm under 150 but be care other ones have come in at around 250 ppm.
Water softeners are to my knowledge really just ion exchange columns that typically catch/trap hard water kinds Ca and Mg and exchange them for sodium ions.
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My point being my concern with my tap water (as has already been pointed out here IIRC) is that I personally have no clue what's the individual ppm of Mg or Ca in my town water. Our town water report simply tells us things like trihalomethanes.
Also my town water has chloramines unlike chlorine ions just leaving it out overnight and bubbling air through it won't do the job to my satisfaction. Hope this helps??
If anything I've said is incorrect or if anyone has anything to add I'd welcome it!!!
Thanks
..... I figure we will just have to wait and enjoy what we do get.....
Maybe use some fine mesh so I'm not farming mosquitoes as well. 
