Hello; Looks like it is made of a form of pressboard (that is wood chips somehow glued together). The earlier of these type of board were poor structurally when wet. I had experience replacing such flooring back in the 70's and 80's. Not sure that the newer stuff which is not designed to be used in damp conditions will be much better.
A TV stand is likely meant to be used in a dry setting, not under an aquarium. It will get wet and may, likely will, stay wet for long peroids of time.
On top of that concern, there is also the likely issue that the unit was not designed to hold 750 pounds. A 75 gallon with gravel and all can weigh that much or even a bit more.
One last thing comes to mind. the finish will likely be damaged from getting wet.
Good luck if you try it