Nitrite should be zero no matter how long between water changes. Far as KH I would bet that is something to do with your water utility or possibly the test strips misreading if they were stored improperly. Its unlikely IMO that a change in KH killed your fish, nitrite and ammonia are much much more likely causes. Only other cause for KH if it measures higher then your tap water is the presence of crushed coral or limestone in the tank.