I don't think you have to worry about a large fishkill. You would have seen more dead fish. How have you been fishing for the fish? They do wise up to your fishing habits. I know that I have more (and larger) fish in the pond than I am catching.
If you want to see what's eating the pellets, go
here and order some. When they come in, rig up your Ultralight spinning rod with 2# or 4# test line, a #10 short shank hook, and a small bobber that is no larger than 1" in diameter. Place the bobber 18" to 24" above the hook. Don't use any weight. Toss it out there 10 minutes before the feeder goes off and start sampling the fish.
When fish have lots of natural forage, they get harder to catch too.