What is your ppm out of the tap?, I ask because if it isn't excessively high and you can get away with a 250l tank, maybe you could just use di. That is what I used to do before I had a driveway, not with the water here mind you, I filled up at my parents house, their water is about 56ppm. A 25l bag of resin used like this would last me 2.5-3 months, and that is with a 500l tank.