Actually, I would consider reducing the amount of food by a little and see if that doesn't help keep ammonia at zero. Ammonia for mature tanks along with nitrites should always test zero. There is no safe level of ammonia that I am aware of.
Could be possible that at some point during the day, such as after feedings,,ammonia levels are considerably higher and that bacteria breaks down the ammonia and by the time you test for it,,the ammonia is at .25. Have seen this happen. You do not indicate what the water change routine is or whether the results posted were before water change or after but with a tank that has been established for the period of time you indicate,, I would expect ammonia ,and nitrites to read zero all day ,every day. You may or may not wish to bother,but in the interest of the fish,,,I might be tempted to perform two or three ammonia test's (5min) to see if levels are perhaps higher at different times of the day.
I do not agree with previous info regarding safe level of ammonia. Usually indicates ,too many fish,too much food,and too little filtration for the fish or bioload present. Opinions vary.
P.S. Clean water means clean of any and all toxins.