My Dad's Koi are becoming more active now, a couple of degrees higher water temps and he will restart their feeding routine. Sadly he found one of his beautiful huge ones dead in the biggest pond the other day.
Sadly that does happen over winter, sometimes and you can never work out why.
I used to put this floating cover over our pool, it's really made for swimming pools, out of heavy duty bubble-wrap, but you can cut it to size. It's supposed to cut out the latent heat exchange between the surface of the water and the wind blowing across it.
I also used to put in a couple of 300w aquarium heater. What I thought that actually did for 3000 gallons of water, I've no idea, "but I felt better for doing ..something."
But I've weened them off it by in one year cutting out the heat and for the last two years not putting on the cover.
So far they're doing OK. But then they're a lot bigger now, some are nearly 2ft in length.
They did take a bit of food, but only a few of the fat pellets I feed to the birds, last week when it was warmer, but have stopped feeding again now. What you don't want is them eating stuff they can't digest when the temperature is low, as it can rot in their gut. But they're fairly active now., rather than being huddled five feet down at the bottom in a corner under the filter return. I've a purgeable bottom drain so the bottom of the pool stays quite clean.
The temperature of the water coming back will be warmer as the filter is in a room in the back of the garage and the filter pump, UV and air pump will be adding some heat. The movement on the surface of the water is distorting their shape. You can also see the reflection of the clouds, It's not mucky water.