Are you buying a house that comes with a fishtank??? WOW....lucky you! Mine just came with a bad roof and crazy wiring...I have a 50g at the moment, in the back of my car with spiderwebs in it...
When i have my own place it'll get buried, filled, and have goldfish in it, as the place i'm buying comes with four or five goldfish in a 10-15g tank, no good!
50g still won't be enough, but it'll beat 10-15g for sure!
My pond is about 2,500 gallons. Comets and fantails, along with a wide variety of "native" inhabitants (mosquitofish, bullfrogs, leopard frogs, toads, salamanders, etc). Here are a couple pics: