Jeebus, people! 
To the original poster: I agree with knox_rbp minus all the arguing with other posters. You would need to check all of the things he talked about before feeling comfortable using it for an aquarium. Even if you can fill it and it holds water, you need to see how thick the glass is and how it's constructed (bracing, top sealing, etc.) in order to know if it's relatively safe. Something can hold water and fail at the first sign of stress (cleaning, a kid tapping on the glass, a large fish smacking the glass, a rock falling against the side), so aquarium tanks are built with a safety factor built in to allow for a bit of extra stress or poor glass quality.
Some pics and measurements would go a long way in allowing us to give you a more informed answer. Things to measure would be tank dimensions and glass thickness for starters.
To the original poster: I agree with knox_rbp minus all the arguing with other posters. You would need to check all of the things he talked about before feeling comfortable using it for an aquarium. Even if you can fill it and it holds water, you need to see how thick the glass is and how it's constructed (bracing, top sealing, etc.) in order to know if it's relatively safe. Something can hold water and fail at the first sign of stress (cleaning, a kid tapping on the glass, a large fish smacking the glass, a rock falling against the side), so aquarium tanks are built with a safety factor built in to allow for a bit of extra stress or poor glass quality.
Some pics and measurements would go a long way in allowing us to give you a more informed answer. Things to measure would be tank dimensions and glass thickness for starters.
