I agree 12x12x12 should be fine from what I've read.
I hate to be the math guy but 12x12x12 is 7.5 gallon roughly. I think the 5 gallon may be a lil cramped but with regular cleanings and such I am sure you could maintain an adequate environment. Especially since some have been successful with Critter Keepers. Apparently that is common in Asia.

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