This is fine, i did the same thing. I moved from a 4x2x2 to a 6x2x2, my joists are soppurting the tank the same way yours is. I would go for more than 12" though because that isnt really much different.
I see alot of people posting concerns about the floors holding tanks, what the heck r they making the buildings from over there? a deck of playing cards? I mean aren't floors solid concrete? Isn't it designed to hold a fridge and stove or a bathtub full of water?
