I'm not sure of an exact answer but about a year ago I moved twice and had to move the fish. I upgraded the tank size each time as well and added "new" stuff to it, plants, driftwood and gravel as well as taking all of the stuff from the old tank. I also added the filter from the old tank as well as a new filter. I did not loose any fish either time and the tank was stable in about 2 weeks both times. Like I said, I don't know all the forces at work, but it worked well so I am a believe in using established media.