Sand Question

I would put the new sand in your tank and let it settle then add the sand from you old tank on top of it...if I understand your question.

 

Reusing your old sand could be dangerous depending on how well you have maintained it. As a rule of thumb I tend not to reuse sand unless there is a way to move the tank without disturbing it too much, otherwise you can release all kinds of nastiness back in your tank. Sand beds can hold quite a bit of junk, once you stir it up it's going to get released into your water which probably isn't going to be good.

If it were me, I would use all new sand along with a cup or 2 of old sand to "seed" it.

 

I have reused old sand with out any problems lots of times you just have to recycle your tank and let the sand cure its no different then adding live rock in my opinion. In fact most of the sand in my current tank came from an old tank and sat in 5 gal buckets for over 2 years

 

Just be sure not to move any tank with a significant amount of sand in the bottom of the tank. I moved my 55gal two times with aprox 60lbs of sand and 2gal water to keep it wet. It worked, my sand stayed fairly undisturbed and the tank didn't need to recycle. However, about 6 months after the second move my tank sprung a leak and needed to be resealed. 60lbs was clearly too much stress on the joints.