If I were your parent, I would want you to come up with a line item budget, both initial cost and maintenance; If the space is really yours, demonstrate it will not interfere with other activities, and emergency plans if something goes wrong(leaks mainly, etc).
I would also want to know if you decide to move out, how to undo these things if the pond also need to go.
I would also want to know if you decide to move out, how to undo these things if the pond also need to go.