I recently purchased a new ro/di unit from http://www.thefilterguys.biz/ro_di_systems.htm (the unit on the bottom w/ 2 di units) and am running into some problems. I recently moved into a house with city water (240ppm) and no water softener and have already burnt up 1.25 di cartridges to make only 100 gallons of water. What would cause the di to get exhausted so quickly?