another option would be to build a raised platform that your tank sits on, say 2-3' in width beef it up so the joist runperpendicular to the joists in your floor, then youl span both joists with lots of support so no sagging.you can even then tile it or match it to what you plan to do with your stand .you wouldnt have to do anything like moving plumbing or ducts and you can still have a normal sized stand
