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"....