I've been gathering the pieces and planning for a set up somewhat similar to what you have in mind with basement sump. Decided to build a bit of a fish room more or less under the area the display will sit. It's about 6' x 5' and in addition to housing sump, fuge, etc etc, it should take care of any support issues that may arise. The floor joist span in that area is 9' & goes from outside wall to another support at the central chimney. Each side of that the span is longer, 11' to a wall that separates the basement roughly in half down the center. Joist support where they meet the chimney blocks looks a little iffy, appears there's just a cross member attached to the longer joists each side of the chimney. This is another reason I feel better about having the extra support the fish room framing will provide. Here's a pic, not great but should give an idea.
The plan is for a custom tank in the 5' range & it will be positioned above the far wall framing in the photo. This is a 5' span & top framing is directly under the joist, so should be well supported. Left side of photo you can just see the last 2x4 of the 6' wall. Anyhow, it appears you're still in the planning phase & with basement sump I thought I'd throw the idea out there for a fish room, two birds with one stone. Either way, I'd recommend some additional support.