turtles will only eat very small fish. they aren't the most nimble of hunters. so anything over 4" should be safe.

The birds on the other hand will go for pretty much any size fish. Even fish WAY to big for them. they will peck and stab larger fish and leave them for dead. I have seen 30+ inch fish killed by herons but not eaten, just stabbed.

Thank you for the info. That darn Heron was out there this morning and I did manage to chase him away..he's huge. All of my larger Koi are accounted for thank goodness. My horse has been wading in the pond and mucks it up..I think this is an advantage to the fish at this time. I guess I will have to just hope and pray! Thanks for the reply as the turtle I thought would have a big advantage stalking under water.

I used to have problems with herons getting at my koi. I got an ultrasound generator box (runs on 110V or battery & costs about $40) that puts out an ultrasound that keeps large predators away. Works great & is very unobtrusive - much better than a net that I had earlier. Bird-X as well as other on-line outfits sell these boxes.

I use ultrasound as well. It seems to work on raccoons and heron. It does not work on king fishers who swoop in and take fish. I still have to have the fishing line to prevent them from eating my fish.

I have seen snapping turtles eat eat fish up to a foot long. I bet they could take them larger with not much trouble. Once while fishing in a large farm pond I heard a commotion on the other side and saw it was a duck being attacked by something. I went to investigate and found a large snapping turtle eating a mallard duck that it had just killed. Don't ever under estimate the power of hunger!
After all these turles are carnivores.
You must also be aware of bullfrogs and water snakes. I once put a big Bullfrog in my pond, and it ate a 10" comet goldfish the very first night. I went out the next morning and there was the frog under the waterfall with a big orange tail sticking out of it's mouth.
I've also seen a few snakes with very suspicious lumpy bellies around my ponds.