Thanks much for a very informative reply. Here is my dilema: I already have bith tanks and a stand. Actually, the 55 has been set up for about a year. I went to the LFS to buy "the largest tank that would fit on the bottom of the 55's stand" and was told that a 40 long will fit. As you have pointed out, while its footprint is teh same as 55, it is significantly lower (probably more thjan 6"). The probelm is that this set up is located in a very narrow hallway, and when I tried to put a 40 on the bottom shelf, I simply dont have the room to manuever it. The solution would be to tear down the 55, pull the stand into the room, place a 40 on the bottom, move it back to the hallway, and replace the 55 on top. I just wanted to make sure that it will work before tearing the tank down.