The inch-gallon rule works pretty well for smaller fish, but once you get above 5 inches, you really have to take into account whether the fish can be comfortably housed. For example, you could never keep a 12" oscar in a 20g tank. There's just not enough room for such a large fish to move around.
For larger fish the tank should be at least:
4 times as long as the fish's length
1.5 times as wide as the fish's length
1.5 times as tall as the fish's length
In this case... 32"x12"x12" or preferrably more, which puts you in the 30g range.
Additionally, an 8 inch catfish is going to pollute far, far more than say... 8 neons. Keeping the tank clean will get harder and harder over time.
That's my reasonably educated $0.02 on the matter at least.