If you wish to use old sand, I would first rinse all the detritus and death out....water running completely clear. Now I would only add small amounts to the existing tank every couple days....the rinsed sand will potentially cycle.
The easiest way to add sand to an existing tank is to take a piece of PVC pipe that is longer than the depth of the tank...top of the pipe is out of the water while the other end is on the bottom. I personally use 1 1/2" pipe. Fill the pipe with sand (I use a wide-mouth funnel), and now slowly lift the pipe off the bottom and move around to distribute the sand. Hope this helps.
