I am upgrading my 20 gallon tank to a 80 gallon tank plus a 20 gallon sump. I am looking into getting an RO/DI water filter, but have read that you lose up to 4 gallons of water for every 1 gallon of RO/DI water. So when I first fillup my tank if I use an RO/DI I would get 100 gallons of filtered water but lose 400 gallons of water down the drain. Im nervous as to how big of a water bill that would be at the end of the month. So is it possible to start with the tap water for the first 100 gallons? Then PWC would be RO/Di.