You want the weight to distribute across as many joists as possible, therefore the tank should ideally be placed perpendicular to the way the joists run.
However, the joists have a definite load limit before they sag. The building code here says this is min of 40 lb/sq ft. If the tank has a foot print of 4'x2' & weight 1000 lb, it is exerting 125 lb/sq ft. Granted, the floor boards, etc, will spread the load over a larger area so the load will be smaller, but I would worry about the floor sagging. The floor prob won't collapse, but who want to deal with sagging floor, cracked ceiling, etc? Better to reinforce the floor, or put the tank right on top of a load bearing wall.
Or maybe ask a structural engineer friend to look ...
80 gal FW with 30 gal DIY wet/dry/sump.
9 fancy golds, 1 hillstream loaches, 1 rubber-lip pleco (C. thomasi), 3 SAEs, small school of white cloud minnows, planted.