I disagree with Steve on this one because you only have 30 gallons or less of water volume depending where you run your water level. But this is just based on experience with more water. It freaked me out at first. I have seen custom tanks that stack on top of each other glass to glasse in which the whole thing was supported just on 2 pieces that were cut 8 inches longer to rest on them. This was a store!!!! This was their display tanks and they were 75 gallons as well. I actually owned a couple of these stacking ones for a while but sold them before I set them up because I had no use for them at the time.
This isn't an argument, or a flame, just an opinion in good nature.
Des