The chance of you pulling out the same water you are changing is incredibly small.....you have water moving through your sump at all times. Somebody did an experiment with this (BRS maybe?) which proved this. I also pull from my skimmer section and return to my return pump section.I also use the Neptune dose. As mentioned it does take a long time. I change out 2 gallons per day and it takes 2 hours. My big question is where to pull water from and where to return it. Right now I'm pulling from my skimmer area and returning it to my return pump area. The dose pumps are set to do this at the same time. Seems like you would be eventually pulling out the same water you're returning. Does that make sense???