Currently I have a 20 gallon tank for my juvenile, but what size do I need for an adult? The pet store people said it could live its whole life in a 20 gallon tank, but I didn't feel too sure about that especially after looking up other people saying they need a 40 gallon. What's the truth behind the matter?
I agree that a 40g breeder or 36"x18"x18" is a min size I'd use for an adult. That's with plenty of time outside the tank. I'd go with 4'x2'x2' or larger (taller) if you can. The more space you can provide, the more variety you can provide which is good.
20 gallon for its whole life.... LoL. Would you like to live in a closet for your whole life or a full size house?
Also in a small enclosure it's almost impossible to get a nice temperature gradient. I would get the biggest you can afford and have room for, but no smaller than a 75 gallon. Mine is in a 125 gallon 4'x2'x2' and he uses every inch of it!