its a fairly invasive thing to do, I did it once in a 55 and what i ended up doing was moving everything into a different tank, (i happened to have a 30 empty at the time) live rock, fish, pumped the water into it then took the remaining water and drained the 55, scooped all the sand out and cleaned the tank, put 25 gallons of new RO saltwater back in the 55, brought it up to temp, then pumped the water from the 30 back into the 55, transferred the fish and live rock back. It was a pain in the *** and the whole process took me about 36 hours but i figured it was better than taking a cup out at a time and causing a mini cycle every time i disturbed the sand bed, this way the fish got stressed only once and i had no casualties except a damsel that somehow made it into the sump with my mantis!