Dawn,
When removing the sand from the old system use a flat kitchen utensil & take the very top level of the sand & use that. That is the aerobic level & very good for seeding the southdown with minimal ammonia spike. I didn't do that when I changed my system over & had an ammonia spike because I mixed the sand from the deep areas (anerobic) with the aerobic & then mixed it with the southdown. very bad!!!! When my tank sprung a leak I did it the way I mentioned at the top of this page. This was done in early spring with the replacement tank. I had no spike at all & lost nothing in the changeover. (other than 70% of my sps due to them being in a tub too long without a skimmer hooked up) They perished in the tub & not inside the new tank. They never made it that far but what did survive in sps were beat up pretty bad & they recovered fine in the new system that was just set up.
OK I am going with a shorter sand bed when we get the drilled tank going I would like to keep some of my old sand bed to layer the top of the new south down what would be the best way to do this? continous ro water flushes,salt water flushes, just put it on top Gods I am starting to hate this 
