I copied a couple similar water change syphon's that I saw here on R2R. It's designed to syphon down to a specific depth before the syphon breaks. This depth represents a specific gallonage. In my tank, 3.25" below the slots in the overflow weir is approximately 10% of the total volume. So, I would turn off or bypass the display tank circulation, let the water level fall to it's non-circulating level, then start the syphon into a bucket (Brute trash can). Ten percent would come out and the syphon will break. Then I just replace the water, using the same appliance in reverse, hooked up to a small power head in my bucket of fresh salt water, and wallah. I turn the circulation back on and I'm done with the water change. I did not glue the fittings on the inside of the tank, so I can increase the depth before the syphon breaks, for example to 20%, if I miss a week. I never asked, but I hope this is how folks are using what I've seen.