eating bacteria has more than likely developed in the tank and saving a bit of the gravel will keep this bacteria alive so that it can resume its duties when the tank is put back together.
use a hose to rinse the gravel. fill the pan with water and use your hands to move the gravel around to rinse them off. tip the pan slightly to pour some of the water over the edge and then repeat.
pour the water off the pan again.
clean the tank and then pour the gravel back in the bottom.
put all the hoses, motors, and decorations back in place and refill the tank with water. make sure 