I don't usually recomend carbon filters, but you might try a home water filter like Britta for a cheap alternative. I'm not sure how much it will improve your numbers, but it's worth a try. You might also consider bottled water.
However you decide to go, don't forget to check and adjust your PPM for your base water count. If your water starts off at 150ppm, and your target ppm is 700, your real target ppm will be 850 (150 base + 700 target).
Good luck!