I started my new 90 gallon tank running with saltwater for about a 3 days. I then started to gradually add live rock from my old tank into my new tank over a span of a week. During this time, I mixed water from my 45 gallon with water from my 90 gallon and vice-versa. I am guessing about 2-3 gallons a day. I then got my ceramic rock into the tank and then started adding snails and crab from my old tank one at a time over the span of another week. I then added some live aragonite sand, which I got from a friend, and then after another couple of days of settling, I moved all my corals and livestock to my 90 gallon.
It's been like this for about a week now and everything seems to be great.
I am still running my 45 gallon with some liverock just in case my 90 gallon crashes.
The only thing I have lost is my rose anemone (pretty sad, since I had him for about 3 years) due to a mishap with a koralia and my blue hippo tang. I had bought a peppermint shrimp a while ago and thought I lost him in the 45 since I never saw him, but when I moved the corals and livestock over, I found him and he seems to be doing great in the 90.
To each his own though.
Anyways good luck with your tank!
chiu