Test the tap water on its own, without dechlorinator/conditioner added, just the tap water. Also, you should be able to check with your municipal water board to see if ammonia is present in the tap water. This would confirm your test results.
Ammonia can be present in tap water, as indeed can nitrite and nitrate. It is wise to test for all three so you know; once should do it, unless there is reason to assume things will change. Ammonia as low as .25 should not be a problem. If the level is higher, you can use a conditioner that detoxifies ammonia, as this will deal with the initial influx at each water change. And the plants (if any) and bacteria/archaea will easily handle it after that.
Byron.
