If your current parameters in the 90 are good, then I suppose it would make things a bit easier to use some of the water to fill the 30. If the water is less than ideal - then don't move the problem from one tank to another.
Rock is fine out of the water for a couple of hours without any significant die-off.
Sand - if you've been in the habit of vacuuming it regularly and your water params are good, you can move some of the sand if you wish - scoop it out of the old and into the new.
If you've had any ongoing issues with phosphate/nitrate, IMO it would be more prudent to lose that sand and get new.
Try to move the sand promptly - right after the rock (put rock in the new tank FIRST, place it on the glass - we never recommend putting rock on top of sand for a bunch of reasons).
If the sand is left in a tank without water movement (stagnant) the bacterial bed dies off surprisingly quickly - so if you can do that within a couple or three hours you'll be fine, but if it sits longer than that, I'd ditch the sand and get new.
If the new tank is being put in a different spot from the existing tank, the whole move shouldn't be too difficult.
Jenn
