when i do a water change,turn of the mainsystem pump.I throw out the old carbon, by that time the last chamber in my sump is full. Then I fill the reactor with the new carbon, put the pump used to run the reactor in the last chamber and let it run clean. I'll shake it up the reactor a couple times. By the time ive emptyed the last chamber of my sump ive also rinsed the carbon of most of that dusty stuff.