When the salinity in the system is lowered initially, it is done as if performing a normal water change, except that the replacement water is simply RO
water or detoxified tap water without the salt (Don Carner, pers. com.). (If the replacement water is RO
or other softened source, then a buffering agent should be employed to prevent pH and alkalinity drops; Thiel, 2003.) Make sure the freshwater you add is thoroughly mixed with the remaining saltwater in the tank as you proceed. This will assure that your salinity/specific gravity readings are accurate. Monitor the lowering closely so as to not reduce it too fast. Achieving the desired specific gravity over a period of several hours is fine (Don Carner, pers. com.). The bacteria colony in the biofilter will survive, the fish will survive, but the parasites will not (Don Carner, pers. com.).
So take out lets say 2 gallons and replace with 2 RO
gallons, same temp.... And watch PH?