Just depends on your view. When I am ready to build my 120, I'll be bleaching and doing the muratic acid on all my rock. Want to start out clean and keep my tank as pest free as possible. Main reason is to get rid of aptasia and lessen the chance of bristles. I know they are good, but my wife hates them.
If it were me.I would buy all new sand, esp since your old sand will not cover the bottom of the 125 anyway. For the price I wouldn't even mess with reusing old sand in a tank transfer or move.
Bristle worms and flat worms are not the same. Bristle worms, IMO, are a great piece of the CUC. Flat worms are a pest, _normally_ you'll see the flat worms as a little red bug on the front of your glass or on the rock. No real detail to them.
CRW Reef;181866 said:
Up to you I guess, only positive thing about using all the old sand in my opinion is cost savings.
I used the same sand from my 210 going to my 120 and don't really see anything wrong with it? I'm letting the 120 re-cycle so IMHO there's nothing wrong with it. Why pay for more sand when I could use the same sand?