Maximum size rectangle that can fit inside a square

Hello all :)

Thanks for all the help ahead of time.

To put some story into place-i am refinishing/rebuilding a 125g saltwater fishtank system. It was a very nice custom built cabinet but came with some very small doors. I want to install a smaller tank in the cabinet below for various purposes but want to maximize the size of the tank.

The cabinet has 2 doors (see here for pictures if you need http://i85.photobucket.com/albums/k6...Tank/tank2.jpg ). As i said, i want to maximize the size of the tank i am putting underneath. I am looking to build a plexiglass tank as large as i can to hold as much water as possible under the main tank.

The window dimensions are 20" tall 16.5" across the top and i would have about 17" of depth before i hit the wall inside. Please dont worry about anything inside as it is currently empty.

Thanks ~!