Well, I always thought 12" was a good size, I suppose 10" being average, but potential to get to 12. I have seen some massive severums at my LFS, they wern't just big, but bulky as well, they had some weight on them. Like I said, if you want to do it go for it, I don't think it would be cruel in that size tank, I mean I'm sure you know what you're doing with water changes, feeding etc. Its just that personally I wouldn't do it.