If you are doing a fish only tank, then you can save some space by getting a taller tank, like the 29gallon. Also, they make a "20g high" which would be okay. Fish prefer width over height but a 20g high would be fine. If you want to add some corals to your tank, you probably want to stay away from taller tanks, as they are much harder to fit enough light over. So, for example, you'd want to go with a 30 gallon, not the 29. But, in general, unless you are actually looking for the challenge of maintaining a thriving very small tank, I'd go with the rule of thumb which is get the biggest tank you can afford and that will fit in the space you have. HTH