I think the best question to ask over here is which salt are you using?
API ammonia will always show a slight green tinted yellow. Plus if you leave the test tube for way too long after mixing the two solutions, if will turn more green, a.k.a give you false reading (I guess the reagent just keeps on building ammonia over the existing ammonia in the water; not really sure about the chemistry since I don't know what compounds are the reagents). I think the best reading for ammonia using API is to do the color match within like a min or two.
Maybe test the tap water directly to see if any ammonia is there?
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