Does it still have the trim on it? If it does it may be a terrarium and not intended to be filled up with water. If it does not have the trim on it, someone may have derimmed it, if that is the case you can get a replacement rim online, you may need to contact an aquarium manufacture but should be able to get one. Aquariums that are designed to be rimless are usually made with thicker glass than what would be used for the standard aquarium of the same size. You can try and add your own center brace like others have suggested, but if it is a terrarium the glass itself is not designed to hold back the weight of that amount of water so it would still be risky. I would not suggest using it as an aquarium. In my area there is a big market for larger aquariums for people to use for larger lizards, you could try and sell yours off and get something different.