Brent's measured capacity for his tank appear to be off from the tank dimensions.
One scenario which would account for his measured capacity as compared to the tank dimensions would be to treat the given dimensions as outside dimensions. The inside dimensions would then determine the tank capacity. Proceeding in this fashion, if one estimates the thickness of the glass end of the tank at 5/8" and the other sides (the lateral sides (circular sides) and the other end of the tank) at 1/2", then Brent's measured capacity is about right.
P.S. 1 ft3 = 7.48 gals (approx.)