I have never kept a freshwater tank. I have however maitained two full blown reef tanks for about 8 years, so i am not new to tank husbandry. I found out today that my wife has volunteered me to maintain the tank at the school where she teaches ( Don't you just love it when your spouse does this to ya.). In topping off my reef tanks i use RO/DI water. My question is, Can i use the same water or does it need any additives or supplements before topping off or doing a water change. The tank has been setup for about three years, is located in the office, and has been under the care of a LFS. They went up on their contract so thats how i got "volunteered". All the water parameters are stable and the fish seem to be doing well. I just want to make sure that I don't kill them by adding water that is " too clean". Any help would be most appreciated.