If I may - a lot of people think high nitrates are ok in FW
aquariums. I on the other hand think not. I do just as much water changes on my FW
tank as I do on my SW
right. It has made a world of difference since I'm not using carbon and really only clean my Eheim filter every 3 months. On my reef I also don't use carbon, but heavy skimming and also weekly PWC
's. One of the plus sides using RODI
water is that you can dial in the PH and GH
of your replacement water to suit whatever you have in your FW
aquarium and keep it there consistently. If you have a unit why not use for both tanks?