Welcome to Koiphen, sorry it's under such sad circumstances. Supposedly garter snakes do eat koi (small ones) and koi will also eat garter snakes in return, if they catch them in the pond!
https://nextdaykoi.com/koi-how-tos/koi-ponds-snakes/
I wouldn't put any type of oil that can leach into the pond too close to it, because if it gets into the water, your koi can die from the "cure". I've thought about moth balls myself to get rid of the rats around my pond
, but am afraid of the toxicity of them. Mothballs are made out of dichlorobenze, and it's very nasty. According to the end all, be all source of information on the internet, Wikipedia says they're not effective against snakes anyway. Not sure about cedar oil; most essential oils in themselves are toxic; aged cedar wood is not.
I would maybe set up a camera to see if snakes are the culprit. And if another fish is harmed, take a picture oft he fish, so we can see the wounds. It can still be a bird, poking through the netting, as it is too close to the water to keep a heron from striking through it. I've also had cats take goldfish out of my patio ponds, years ago, and I think a cat got one out of my pond over the winter, as the fish disappeared (small, maybe 5-6") and I saw a cat inside my enclosure. Never found the fish, just a few scales.
How deep is it, and how many gallons? Nice looking pond and fish, I hope the attacks stop.
Nancy