If it there was only 18 gallons for the fish to swim, that would mean that the back would take up 1/3 of the space.
I know the 29 gallon is 20.1 inches long, and 19 inches high. I just don't know how deep it is. A gallon is 231 cubic inches, so If we assume 3 inches, then the back area is about 5 gallons, leaving 24 gallons. If its 4 inches deep, then it takes up 6.6 gallons, leaving a bit more than 22 gallons to swim in.
I haven't seen a 29 gallon biocube, so I really don't know how big the back is. If your LFS has one, measure it, and you will know exactly. And of course, you will have sand/gravel, rocks and plants, so its a bit less.