If the tank was being supported just by the posts, either way would work. But, since the tank should be supported around its lower plastic frame, it is really the horizontal "beams" around the outside that do the supporting. If you make that frame with good enough corner joints I think you could make it work with the blue post, but it would be much easier to make and make it structurally sound if you use the red posts. That way each "beam" is supported at its ends.
My thinking with the blue post is that it, combined with the 1/2" plywood on top, would make the front corner joints not as critical for weight bearing, as the three corners and the entire front beam would be the main weight bearing points, with the other short beams being there more for torsional support. I am not an engineer, though, so that might not be accurate. As for corner joints, I was intending to bevel-cut the front piece with 45° angles and then drive two 3" deck screws into the ends.
Why not use the red posts, but make the door in front be a two piece door, with each half hinged to open from the outermost black corners. The doors would then be bent to fit around the outside. It would look great, and have good access too. You could also add a second hinge to each door, like a folding door, so the hinge in the middle makes it bend around that post.
For access, I was planning on making removable face panels instead of doors for ease of construction and because of the tank's future location. With the red posts, I'd have about 9" of access width on the square sides and 6" on the diagonal front, and the posts would almost be completely under the front beam anyway (barring the use of 2x2s or bevel-ripping a 2x4). With the blue post, I've got two 14" wide accesses instead, which makes 2-handed access a bit easier. For size reference, the two long sides of the frame would be 22" long (same as the long sides of the tank).
why not use a sheet of plywood for the top and top frame two layers of 3/4 inch should make a strong top to connect the uprights too. then you don't have to worry about any joints that need support. then you would be able to do just 4 uprights, and still make the bi-fold doors for great access.
Redman, I'm not quite sure what you're describing with the double-layer of plywood...do you mean to use a double layer of 3/4" for the top and connect legs to that? If I've got that right, I'd be worried about twisting under load. I'm using BiscuitSlayer's 75 gallon stand build as the guideline design for the frame on this build.