My wife asked me this question and I did not have a good answer. Perhaps yall can help me out. When I do a water change, I put the tank water I remove down the drain. She asked me if I couldn't recycle that in some way. It seemed to her a waste of salt, which can be a bit pricey. At first, I just said "No" but I couldn't give a valid reason. If you had a large barrel with a couple of hundred pounds of live rock, some powerheads and a light source, could that clean up the water before your next water change allowing you to put that water back into the tank as clean? Am I missing something here?