I don't build tables, but this is my understanding of the situation. In the first place, we can accept that the tank isn't set where the stand designers rated it to work. That doesn't mean it will fail immediately, soon or ever. It does mean that failure is more likely than had it been set properly, but we don't how much more likely it is.
It's difficult to say without knowing how the stand is designed. Per your post, it's basically 4 pillars with 4 connecting beams (the "rectangle") covered by a table top that extends beyond the rectangle on all sides. The top could also be essentially a single welded piece of metal.
Normally, I'd want the top and the beams to be acting in concert to distribute load directly to the pillars, however they don't have to be designed that way. In one case, the beams could be primarily for stability. If the table top actually has only 4 points of contact and those are the pillars (legs), then the beams aren't carrying much load (if any) and the table top is meant to do all the work of distributing the load evenly. On the other hand, if the beams are on top of the pillars or set to run at the same height as the pillars (and possibly reinforced with angles from the pillars), or if the top is a single welded piece sitting on top of the pillars, then the beams are engaged as well in taking load and transferring it to the pillars. This is probably how it was done and the beams are therefore distributing load.
In the latter case, you probably are fine as most of the tank's load is being carried elsewhere. On that questionable edge, the beams and table top are being subjected to some "extra" shear force that could have been avoided by proper placement, but it's probably small as the tabletop is conveying a lot of load across all of the beams.
In the former case, the tabletop alone is taking extra shear force and then it might depend on the strength of the table top. That doesn't mean it will fail, since the force is being directed to a relatively small area.
:thumbup:I'd love to see that once finished perhaps you could post it in my artwork thread 
