I'm sure there are as many different justifications as there are fish-keepers.
If you're going to do it, do it right.
I've already spent $XX, what's a little more.
My fish really need the extra space.
My spouse is away for the weekend, they won't notice.
My corals really need better light.
I'd probably just spend the cash on drugs/alcohol/sex or all three, not necessarily in that order.
Etc, etc....
The list goes on and on....
