From the condition of the fish reported
and the water level going down I wonder if
a raccoon has been making the pond a nightly stop?
If so you can try these deterrents:

~ An electric fido shock fence, found in most large
pet stores, can be put on a time to avoid shocking ponder
and pets.
~ A motion activated sprinkler
~ Call animal control and ask if they have a Hav-a-Hart
humane animal trap to borrow or rent


kathy :-)
A HREF="http://www.onceuponapond.com/"Once upon a pond/A