i'm in the same place as you are keish, and here's what i did... i was supposed to upgrade from a 55 to a 125 yesterday, however my LFS supplier was out of stock so i have to wait till monday... However: I took all of the water, LR, and livestock out of my 55, along with my skimmer, heater, and PH's, and put them in an extra 55 i had laying around. I then filled up the 55 DT with newly mixed water, and stirred the sand bed up super good to release all the detritus, and whatever ammonia, nitrites, or nitrates may have been stuck deeper in the sand bed. Then I added another 2 bags of sand in with whatever sand i had in there and mixed it together. i added a small piece of LR to it, did a few 100% water changes and it worked like a charm. so if i were you: i would move all the water, LR, livestock from your existing 55 into the 90, then add water to the 55, stir up the sand bed..yada yada yada. The reason i chose to mix my sand, by the way, is because i want to make a bare bottom QT out of my old 55, and i saw no reason to waste the sand i already had. hope this helped you out. good luck
