Like Jake and F-Rod said, I would drain the old pond and kill off or 'relocate' any existing fish.

I would think it would be good to drain the old pond and let it dry out so you could go in and clear away the decades of muck and silt build up.


Excerpt from Robert Crais' "The Monkey's Raincoat:"
"She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."