Bleach in pond

My aunt has a 1250 gallon pond with a skimmer and UV light.
Apparently her UV light went bad and her pond turned into
pea soup. All of her fish died, I suspect from lack of
oxygen; but they assumed it was the algae. My well meaning
but misguided uncle poured a gallon of bleach in to kill off
the algae (at least he took the plants out first.) It
worked, not a hint of algae in the pond, LOL. What will
have to be done to make this pond fish safe again? Will it
have to be completely drained and scrubbed? Would a couple
of near total drainings and refills followed by
dechlorinator work? Ideas?

Robin
http://community.webshots.com/user/robinandtami