Old sand to new tank

Hi sorry for hyjacking the thread, I'm going to be upgrading to a 90g soon my old tank would be about 6 mounths old why could i not take the top 1 inch and not use that but use the bottom 1 inch?

I mixup my sand every time I do a water change, by that I mean that I use a siphon to clean the sand. So definitly the top inch or so is clean every 7 - 14 days, but that is where the ditris sits and I can see that to use it you should boil and rinse very good. The bottom 1 inch or so is where most of the good stuff would be anyways as long as it is not suficated by the ditris. I have taken sand from my DT and started a new sand bottom in my daughters tank a month back with no troubles.

Bill