I would not recommend using this table for a fish tank. There are too many potential sources for failure.
- If the tank rests upon the 3/4" top instead of extending to the width and length of the metal support frame, the risk of failure increases due to the potential weakness of the MDF board itself.
- The ability of the metal apron to support the load depends upon the gauge of the metal. If it is too thin it will buckle. I am not aware of any rule of thumbs that could easily predict the carrying capacity of the sides.
- It is not clear how the metal aprons connect to the legs. If the legs are not fully welded or otherwise adequately connected, then any sideways force on the tank might cause the entire the leg/apron connection to fail and the tank collapse.
- The legs are very slender and will may be at risk of buckling under the load. Since the table does not include a bottom shelf, there is no additional bracing to keep the legs stiff and vertical.
- Each leg will transfer approx. 150 lbs. to the floor in very small areas. This may be a problem for a wood framed floor and may leave a permanent impression of the finished flooring. A tank cabinet would spread the weight over an area of flooring better than these slender legs.
Since you asked this question in WSE, I would suggest researching commercially available tank cabinets and possibly making one of your own. If you have questions as you plan the base, this site is always available to help.