[Agu]

Put the old sand on top of the new sand.

Depending on how old the tank is and how the sand smells you may or may not want to reuse old sand. Personally I replace almost all the sand when moving to a new tank and just seed it with a few scoops from the top of the old sandbed. Then I go through the old sandbed picking out any worms/ critters from the old sand to transfer to the new tank.