i'd have a huge collection of colourful, freshwater, comunity fish (incl. some bristlenosed catfish i just luv them!), and have the tank about a foot thick and have it as a sort of dividing wall in my house, not quite sure how i would clean it....... oh well it's still in my head, and as long as it stays there i dont have to! 
Or a breeding colony of leopard frog plecs. Hang on, you could fit a pair of royal farlowellas in there. OOOh yes, it would be the farlowellas 
