I think I know where you have got confused. Are you by any chance calculating the exact size they have given you for the aquarium? I.e. length height and width. What you've got to remember is that even though the aquarium might measured 24 inches high, the water won't actually come up that high inside, it will probably only come up around 22 inches. That will make a lot of difference. For example, my tank 6 x 2 x 2 if you work that out, it is something like 150 gallons. However, the water only comes up around 22 inches which actually means there is only around 125 gallons of water in the tank. I hope that makes sense now