... also known as carbonate hardness or total alkalinity.

Technically, KH is the measurement of the carbonate ion in the water.  These carbonate ions are neutralized by the acids produced by your biofilter when it eats ammonia and nitrite.  These carbonate ions are the KEY to keeping a healthy pond.  If their level goes below 50 ppm, your biofilter is either struggling to do its job or it's dead.  The ideal level is between 80 and 150 ppm.  Bead filters (especially during their first 6-12 months) function better at a level of 200-250 ppm. 

Some water sources have an ample supply of KH.  Most don't.  Test your water source also to find out the level of KH you're adding to your pond when you do water changes. 

Raising the KH in the pond is relatively easy.  You add sodium bicarbonate (baking soda) dosed at 1/4 cup per 1000 gallons per day, testing daily, until you reach your desired level.  When your KH tests over 100 ppm, your pH fluctuations will be minimal, your biofilter will be working efficiently and your koi should be happy.

 WORDS OF WARNING while increasing the KH

If you're showing elevated ammonia levels, you MUST use an ammonia binder.
If you're showing elevated nitrite levels, you must add salt to the pond dosed at 0.10% pond volume .