so far i added about 40 gallons from my 75 gallon tank into my 125 and added about 40 gallons of new water. i moved about 60 lbs of rock also and everything seems fine. fish are swimming and eating. i'll top off water this weekend and then move the rest of my rock which is about another 40lbs. i'm still iffy on the sand. i know there is a big chance of cycling if i add the old sand. i'm thinking of adding just a lbs every few days and measure. what do you think.