Use old water or start fresh?

[ScubaSteve]

Trust us, you want new sand. When you transfer that tank and stir the sand up you'll see why. There is all sorts of funky going on in there. You might find some of the areas buried under rocks has become anaerobic and smells like rotten eggs. I find I feel just gross after handling old sand.

Sand is like the kitty litter of this hobby. Everyone poops on it. It's gross.