75 is the same length and height as a 55. It's just 5 inches deeper (sits 5 inches out further from the wall). You've got these things to consider. The weight of the empty tank and stand. 1 gallon of water is about 8.3 lbs. 55 or 75 pounds of gravel, plus your decor, filters, etc. How old is the place it's going in? I would think most anyplace would support either of those tanks, unless the place is extremely old, or has an extremely uneven or creaky floor.