Your tank really isn't fit for any predatory bottom dwellers, with the exception of bumblebee gobies.
Edit: And I didn't really think about it before, but if your tank is 2' tall by 1' in diameter, it is nowhere near 40 gallons... those dimensions would probably put you somwhere around 15 gallons... but anyway, if you are going with the puffers, be sure to read up on them first; they are a bit more needy than most fish. I would recommend taking a look at http://www.dwarfpuffers.com/ This is the website I used when I was considering getting into dwarf puffers (I ended up deciding I would prefer to have a community instead).
Edit #2: Actually I just did the math on it, and according to those dimensions, your tank is a 10 gallon (9.7 gallons, to be exact).
Edit: And I didn't really think about it before, but if your tank is 2' tall by 1' in diameter, it is nowhere near 40 gallons... those dimensions would probably put you somwhere around 15 gallons... but anyway, if you are going with the puffers, be sure to read up on them first; they are a bit more needy than most fish. I would recommend taking a look at http://www.dwarfpuffers.com/ This is the website I used when I was considering getting into dwarf puffers (I ended up deciding I would prefer to have a community instead).
Edit #2: Actually I just did the math on it, and according to those dimensions, your tank is a 10 gallon (9.7 gallons, to be exact).