I'm betting the ec out of the tap is pretty high for you. For me, 10g of ro water (38L) only takes 2ml of ph down to go from 8.0 to 5.5ph. If I have 10 non-ro gallons it takes a hell of a lot more to get to the same ph, and it's about 300ppm prior to running through the ro system. ebay ro filters are around $100 or so, and the one I have works great, has filtered A Lot of water and still reading 0-10ppm after all this time.