I certainly hope that you plan on putting a full top on that stand and a support in the middle of the top of the stand. That tank requires full bottom support, and a layer of foam between the tank and the stand top. That tank, cannot be supported in the same manner as a rimmed glass tank.
I am using the mat that I purchased with the tank, but wasn't planning on any center support.
Why would it matter if it is perfectly supported around the outside perimeter. Are you thinking that the bottom pane is going to flex and pull the exterior seems?
I could and thought about using a 3/16 inch piece of plate steel to distribute the weight but thought it might be overkill. Notice the leveling feet and it was welded then machined flat within a couple thousandths. But i look forward to opinions.
The bottom of the tank takes the full pressure in the tank, if not fully supported, it is going to deflect. To adequately support the tank, the tank needs one of two things: 1) a much thicker bottom panel or 2) full bottom support. Lacking one of these two, the tank will self destruct over not so great a period of time. This is a basic rule concerning rimless glass tanks, it is not really an opinion. It can be supported in the shown manner--IF the tank is built sides around bottom, and the bottom panel is raised up inside the sides, thus the weight of the tank is supported by the bottom edges of the sides very much like a rimmed tank. The bottom panel must be thicker, though, to keep from self destructing.
I am not sure what the mat you purchased with the tank is, but you need a 3/4" ply hardwood ply top on that stand, a a rigid foam pad to prevent point loading. I also strongly urge you to have a center support welded to the top of that stand. With a living room full of water, it is a little late to say "oops"....