I add about half a gallon of RO freshwater daily to the sump of my 90 gallon saltwater aquarium. I drew a line with permanent marker to indicate the appropriate water level. Sure enough, the level goes down about a half a gallon daily. Why after a week is my water considerably saltier. It seems to go from 1.025 to 1.03 in a week even though I'm keeping the water level the same by adding the freshwater. Does it have something to do with fish respiration/digestion/something else?