I would say that you'd need at least a 20 gallon for a snapper that size. They are really good swimmers and would probably appreciate more room. Also, they will need a basking spot(totally dry land or rock area). Common snappers can grow fast so be prepared to have a 55 gallon or larger tank to move them into in the future. Right now, I have my year-old common in a 125 gallon tank and his carapace length is 7 3/4". You can see pics of him at http://photobucket.com/albums/y224/turtleman85.
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1.0.0 Chelydra Serpentina Serpentina --- Snappy
0.0.1 Clemmys Guttatta --- Speck
1.0.0 Graptemys Pseudogeographica Pseudogeographica --- Chomper
0.1.0 Eublepharis Macularius --- Sabrina
0.1.0 American Staffordshire mix --- Daisy