Dear MDs: Although my daily diet is > 5000 mg of K rich food/fruits, my K is <3.4. By how much would 12.5 mg diuretic (hctz) decrease potassium level?

Can be significant. HCTZ (hydrochlorothiazide) can certainly do this! Also contributing can be subtle things like diuretic effects of alcohol or caffeine, even large amounts of licorice! Some forms of hypertension are associated with low K to begin with ("hyper aldosterone states") and are very sensitive to even low doses of HCTZ (hydrochlorothiazide).