Buy a Merlin ro samll enough to fit under sink,in garage,and can produce enough water in 6 hours to fill a 1000ltr tank
You will still need a resin di for polishing down to 000.A merlin will now only cost you under £300 they were and some still are over $400+
This is the cheapsest way and i have saved over £4,000 in resin over the last 8 months.A 1000 ltr tank will cost you app 0.98p to fill including the watse.
5 days @ 0.98p=£4.90 x 52=£254.00 per year not bad considering how much you can turn over in a year with a full 1000ltr tank every day.