Is that an API ammonia test ? If so, be aware, it does not distinguish between free ammonnia, the dangerous one, and ammonium, the chemically bound one that is not usually dangerous.
If you have a reading of .25 or less it may not be free ammonia. API shows only the total of all ammonia, regardless of form. If you have nitrites and nitrates and still get that low reading, it's likely ammonium that is the cause.