Is this a bad idea?

[sphelps]

Originally Posted by Aquattro

The sump won't fit all the rock from the 90, but I'll put 40 or 50 pounds in. The trasfer might take a while as I'm cutting all the corals off the existing rock and remounting to the new rock. So I have to at least keep the 90 3/4 full and just use the water from the sump.

Yeah you should be fine if you can use as much water and rock as possible from the 90. The rock is obviously more important since the water actually has little bacteria in comparison. Is there any reason why you can't use more rock from the 90 and less from the bath tub? Also maybe you can move some of the corals with the attached rock to speed things up and then as time goes on you can do a little rearranging in the 180 when things are 100% stable.
The faster the better IMO and I would spend your time and focus on the 180 rather than trying to maintain the 90 as well.

If it helps pretend your 90 gallon is going to explode in a the next few days and you're not sure exactly when, could be tomorrow or next week. Better work fast!