Ultraviolet Clarifiers should be turned off whenever the pond water is showing any signs of ammonia or nitrite.  They don't discriminate between killing good or bad bacteria, so they prolong the nitrogen cycle.  An all too common occurrence in newly set up ponds.  

Water parameters should be checked at least once a week through the winter months.  
A lot of people tend to "let it go" at this time of year.  Koi are cold blooded animals and their metabolisms are slowed down through the winter.  But, they still produce ammonia --- just at a slower rate.

 

Water is heaviest at 36* F.  You can have 38* water on the surface and you can have 34* water (or ice) on the surface.  When this becomes important is if you're pumping water from the bottom of the pond if the water on the bottom is warmer than the water above it.  You're cooling down the koi's comfort area in the pond.  If you're going to pump water for whatever reason through the winter, you should pull it from 8 to 12 inches below the surface and return at or below the water surface. 

If you have a sufficient snow load on top of the ice it will block the sunlight.  This will cause the algae to either use oxygen and produce carbon dioxide (all plants do this at night) or die --- creating an ammonia problem.  Either way it's not a good thing.  You should remove enough excess snow to allow sunlight into the pond.  Water sprayed from the garden hose does this pretty well.

  Koi DO have teeth.  They're in the back of their throat and used for grinding.  They lose them occasionally  as they're replaced with new ones.  These were scavenged from the filter through the summer.

Salt water is heavier than fresh water.  It can form layers of different densities in your pond if there isn't enough mixing.  NOT a likely occurrence, but it can happen.  When testing the salt level, it's a good idea to pull a sample from the bottom as well as the top to confirm proper mixing has occurred.  This is more likely to occur in a wintertime scenario where the water is being pulled from just below the surface and there aren't large enough fish to sufficiently agitate the water.

1 ppm ammonia converts to 2.7 ppm nitrite, which converts to 3.6 ppm nitrate.  
From our experience, water changes have proven futile to dilute nitrite.  More frequent water changes to dilute the ammonia to less toxic levels at the beginning will be easier on the fish.  You will still have your nitrite spike --- but you can easily weather your koi through this with pond salt.  Water changes EASILY reduce nitrate levels.

Koi's immune systems shut down for all practical purposes in water temperatures below 55* - 60*.   Harmful bacteria (aeromonas and pseudomonas) are active in water temperatures above 50*.  When water temperatures are between 50* and 60*,   koi are easily stressed.  Maintaining ideal water quality helps to ease the stress.