Exterior Walls & Weight Question

[Chin_Lee]

I joists are quite strong already and the front of the tank (which will be over the foundation) will be pretty strong. Its the back half of the tank that would be of some concern. If i were you, I would get a couple of 12" radius cement tubes and pour a 2-3' column of cement at each outisde corners of the cantilever. set in the 4x4 or 6x6 post holders before the cement dries, and then support the corners with 4x4 or 6x6 posts.
BTW - that would be an ideal location to build a fish room in the basement underneath your dining room where you can plumb the upstairs tank into a sump in the basement. If this is something you want to consider, i suggest pulling through a couple 1.5" and one or two 1" spa flex pvc tubes through the joists now. Or plumb regular PVC through the space between your exterior wall and gyproc and into the basement.