Use old water or start fresh?

[The Grizz]

I have move sand from 2 - 65 gal tanks in to my 165 gal after washing, transferred ALL the old water & rock topped it off with new water to desired salinity, let it all settle for a day & in went all my fish & coral, didn't lose a thing.

Also did the same with my FOWLR. I think the key is to use the right thing to wash the sand, I used a fairly large fish net with very small netting. Washed a couple cups at a time in water as hot as I could handle.

Not going to say it's the right thing to do but it has worked for me every time I have moved a tank.