Ok I know some off you may cringe when you read this... Sorry to all those..

Others might feel like calling the RSCPA! Carry on, do it
I've been turning my pump off through the winter months for the last 8-10 years.
I'd say my pond gets at least 3-4 months a year with no pump on and no UV on.
During the extreme winter they also don't get fed at all.
I have koi now that I had since I had my first pond, bought them at 2-3" and there now big and healthy as any Koi I've ever seen.
I would say my pond being more of a natural type, with plants and a bit of algae would always have above and beyond the amount of bacteria alive and kicking because I haven't ever overstocked.
I would never claim to be any sort of koi expert, when I started out keeping them I learnt purely form word of mouth, no wonderful internet back then!
The only times I've ever lost fish was once when my largest ghost koi was egg bound and unfortunately she died. One time one jumped out and was found laying next to the pond. I did lose one that can't be explained as it simply vanished and no sign of it (I put it down to being nicked by some scallywag).
The only time I've lost fish to talk of, was during my house moves. None ever died during transportation, but did die during there time in other ponds.
This was while they were being looked after by the kind of keepers who had really posh setups and did it all the "right way" with all the treatments and adding this and that to the water. I've always found my losses are minimal to the point of zero. Having spoken to other guys who would call themselves "serious koi men" I've lost a lot less in comparison then any of them.
I wont change what I've been doing because all my fish have always been healthy and I've never had any problems *touch wood*. Bar the egg bound incident when the fish did need treatment, but still died I've never lost any coldwater fish to illness. Anyone is welcome to come and view my pond any time and I promise you, the fish are as healthy as you'll find anywhere.
Of course I'm not saying my way is the right way, I'm not saying other people should do as I do. But I think my losses and the general health of the fish speak for themselves.
Let's not lose sight of the fact that many koi imported to over here are bred and grown out in things that resemble swaps morethen garden ponds!
I would advise any koi keeper or keeper of any coldwater fish to have algae. Don't go down the road of the super clear no algae pond, it might look nice but is it natural no.
Each to their own but I stand by my methods of keeping my coldwater fish.