I get all my water from one of the local "Filtered Water" businesses. Not the do it yourself machines you see in Safeway but a proper water business that refills the blue water bottles for household drinking. This all depends on the size of your tanks, two bottles last me 4 weeks.
Ask to see their logs, they should have a log book of TDS going in and TDS at the end of the pipe. I am lucky in that I found a new guy in Surrey who knows his business and I get as pure waster as you can get without distillation.
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