Hmm.. I am not as experienced as others' on the forum.. so hopefully someone will correct me if I'm wrong - constructive criticism always helps.
So, KH is a measurement of carbonates and bicarbonates in your water.. which, to put it plainly, are various minerals present in your water (minerals that contain the carbonate ion, bicarbonates are simply more acidic carbonates - you could say). There's not really a bad amount of kh depending on the type of water you need to keep as it affects GH (this is because KH is a small part of the GH measurement - which is your general hardness, basically calcium and magnesium ions). KH is really just your water's ability to keep ph stable - which is important for your fish. Sort of like a car bumper, which takes damage in an accident instead of your cars frame - your KH does this for your PH.
So if you are experiencing swings in kh: it likely stems from your maintenance schedule/water you're changing with, or things present in the aquarium (such as: substrate, shell decorations, crushed coral, mineral supplements, shrimp/crayfish husks, cuttle bones, etc. etc.).
Questions for ya:
Is this about a freshwater tank, saltwater tank, or brackish water tank?
What is your kh, and what has it been doing? Can you provide details of how wide the range is?
Also, what is your ph & gh?
What kind of fish/plants do you have, or plan to buy, for this tank?
And, what kind of additives do you use with your tank? Have you been adding anything to try and manipulate your kh?