Ahh. Then usually it is good to wash, as it removes any crud that can be stirred up and then decay. Not so bad when just sitting there, but when stirred up it can decay and spike nutrients, but the worst part is it can settle on rocks and clog the pores in the rocks.
I did this for a reason.
First, there is more in old sand than dead and decaying matter. All of the micro-organisms that covert that into healthy water are present in that sand. To wash it out removes the "gunk" but also kills or removes that microfauna.
Second, I ran my existing canister filter on the upgraded tank for over a week. This not only cleared out the gunk from the water column, but also kept in the system any and all of the microfauna that I believe kept my 55g system healthy and beautiful.
Did I make a mistake? Time will only tell. Aside from the film algae issue, Which seems to be most prevalent on the glass, after 18 days, I see no other algal blooms.
I guess you could say I am running an experiment as to the value of washing or not washing sand, both new and existing.
As a point of reference, the 55g was started using a 5g bucket of used UNWASHED sand from the guy I bought my 180G tank from.
I am strongly encouraged to believe that there is a considerable degree of quality in reusing old sand than is given credit in the hobby.