Hmmm. I agree 6 inches is tight. My smallest clearance is 7+ over the 90G. But there's another 3 inches between the rails of the support for the tanks over it, so it's at around 9 maybe 10 usable inches directly above the water line. Are you measuring 6 inches between the tank rim and the bottom of the front rail of a stand or shelf? Would there be more working room directly above the water?
Wait. I lied. I have 3 29G's under the stand for the 125 that only have 4 1/2 inches between the trim and the bottom of the rail. But the stand is a bit of a beast and there's another 8 inches above the water. It's a bit of a trick to get a specimen container in there, but it works. They were set up to be tanks that I wouldn't be having to dig into very much. Gotta make the best of the space you have.
"Why can't my wife see all of this stuff as an investment?"