My 1200 gallon pond in Sacramento California has a few Kois in it
since I put them there last spring. So far they have been very happy
and voracious. Since the weather has turned cold, and particolarly in
the past 7 to 10 days, I noticed left over food pellets sitting in the
pond overnight. At feeding time they do not come to the surface
anymore like they used to but otherwise I am not seeing any other
signs of distress. At times they are vivacious, but generally they
seem to be staying on the bottom on the pond in schools and they seem
suspicious of me approaching the pond, in a fashion similar to the one
they had after I cleaned the water this past summer.

I have been feeding them the same amount of food and the water is very
clean. Nothing has really changed aside from the temperature outside.
Should I be worried? Is there anything I can do to find what is wrong
with them?

Thank you in advance for your help.