Simple sad truth, the quickest way to kill a Koi is to name it.
Predator, disease, equipment failure, freak lightning strike, or some combination of those will target that most favored fish. To this day, I'm still half-convinced that it is some sort of conspiracy against those who would keep fish.
My first encounter with Birdzilla was when one systematically killed about 30 young Ogons, about 10 to 12 inches in lenght. In-ground pond with a nice stream between the filter section and the main pond. I added marginals along the edges and a mulit-strand fishing wire fence. The provided only marginal protection. Big Bird doesn't like landing in water, they prefer to wade in. If that isn't do-able, they are perfectly happy fishing from pondside. And to make things worse, they will spear and kill fish that they cannot possibly eat.
One Heron managed to tangle a wing in the fishing line. It was a bit tricky, but I managed to clip him free so that he was able to fly off, uninjured, without any of the line still on him. (I've seen what fishing line can do the Pelicans in the wild.) Happy to say that this bird never did return.
This encounter set me off on my path to building landscape timber and plywood ponds, half in-ground, half above ground. They were small, averaging about 1,200 gallons including the filter system, but at one time, I had 4 of them, and some large stock tank ponds, as well. The above ground ponds had about a foot between the water level and the top of the pond. A raised netting frame kept the net about a foot, to 18 inches about that. This was high enough to prevent spearing attacks.
With larger raised ponds, those with edges well above the water level, I'd planned on a short fence on top of the edge that leans out at a 45* angle over the water to keep be buggers at bay. This type of netting would do the job, without being much in the way of an eye sore.
But, Life is simpler for me these days, as I'm not keeping Koi. Just a couple of goldfish in each waterlily pond, bought as a bag of feeders. Later on, I'll be building a Koi pond and probably find myself doing battle with the feathered adivisary once again. I do love seeing those winged beasties, just not feeding on my fish. I still laugh at myself when I glance around the back yard and my heart nearly stops because I think I've spotten one on the ground.
It's only the plastic Heron "decoy" I put out there after someone said they were supposed to keep the real ones away. Didn't work, of course. I've actually seen larger numbers of the Great Blue standing shoulder-to-shoulder on the bank when baitfish were running. Just like fishermen will do at a favorite location under the same circumstances.
Not sure why that decoy is still out there after all these years, but it is. And I will probably leave it there.
Mike S.
Spring Hill, FL