The thought occurred to me, and I'm not sure if I've positively identified a reason this couldn't work, so please, someone give me a good reason! My thought is why couldn't I run a stocked freshwater tank with good filtration as my ATO, then just refill the freshwater tank with RODI water as a partial water change? Are there bacteria that occur in a freshwater tank that would/could hurt the saltwater tank? Or would the bacteria just die, causing continuos ammonia problems? I feel like this is one of those things where if it worked, someone would have already done it. I can imagine some pretty cool ideas for set ups, like a species specific tank hosting macro as a 'fuge on one side of the DT, and a freshwater top off tank on the other.