The Evil Greebo is correct. Glass is simply not that strong. So if you intend to do this despite the warnings you will get, I'd find a glass shop that can supply you with a replacement glass top that is thick enough so it WILL support the design load. They may suggest the use of tempered glass. The problem is, thicker glass will be fairly expensive, especially tempered glass.
Another option, coming at this as a woodworker, is a way that is commonly used to strengthen long shelves that are supported at the ends. Essentially one would laminate a second piece of wood along the front and rear edges of the shelf. This creates a beam that is stronger than the single thickness of shelf material, with less deflection. One would probably need to use an epoxy to form the lamination. Again, a glass shop would tell you if this was feasible or worth the expense.
Finally, an additional support in the center of a shelf will vastly increase the load it can support, compared to a beam that is supported only at the ends.
To be honest, I'd go for an appropriate thickness of tempered glass. It won't be cheap. Even better is to avoid the glass top completely. Remember that failure in this case is an extreme event. Lots of water, dead fish, big bills to clean it all up.