Tinkster,
I hope you don't beat youself up over this, it does happen, I've been through it myself, it has probably happend to more people than they'd care to admit to.
There has been some good advice in the post above on how this can be made less likely to happen. I'd be surprised to find that advice came from someone who learned the lesson the hard way. That's how I learned the lesson.
It helps when you can decide just what the primary function of the pond is. Is your primary interest in the plants, or the fish? When you can make that decision, you can do a better job designing a pond, or maintaining an existing one. I don't recall seeing too many backyard ponds that wasn't over-stocked with fish, if there was fish in them. It is that over-stocking that usually sets up this sort of disaster in the first place. The the tendency to over-stock is quite natural. If we followed Nature's stocking rates, many of our ponds would have NO fish in them, thoses that did would have so few they'd be hard to see.
When we increase the given body of water's capacity to hold fish, we also decrease the fish population to survive any kind of interuption of those "improvements" we made. Comes down to a simple equation, as the number of fish in a fixed volume goes up, the potential for loss goes up, exponentially.
Or more simply, fewer fish, more water, less problems.
A "properly stocked" Koi pond, usually reserved for high-end, show quality Koi, might only have one adult Koi per 1,000 gallons. At that stocking rate, the fish can comfortably survive for days with the power shut off. That sort of stocking rate is not something you are likely to find in a watergarden.
When you are keeping both plants and fish in the same pond, you have to compromise between the needs of both in the basic design. If one is more important to you, then you make that compromise lean towards that side of the equation. It's a balancing act. And there is nothing wrong with that.
You sound like the type that will learn from such an event, and move on. It's the best thing to do. I sincerly hope that's the way it goes.
Mike S.
Spring Hill, FL