I agree with Pottsy. The film may be natural and not related to the dead fish. I have a trout pond that gets a yellow film etc on the windward side occasionally and the trout are doing fine. I believe mine is a combination of pollen and waste products from the fish feed.

Have your fish recently spawned? Are there a lot or just a few? Some fish get so stressed from spawning time that some die. Or is your pond high in organic material i.e. dead leaves mucky bottom etc., and you did you recently have a turnover from a warmup and then heavy rains?

If pigs could fly bacon would be harder to come by and there would be a lot of damaged trees.