Another test kit in which Salifert stands alone is the activated carbon test. In this test the activated carbon itself is tested on its remaining capacity. Thus allowing hobbyists to decide when the activated carbon has to be renewed. It should not be neglected that a ‘old-fashioned’ criterion of yellowness of the water for pinpointing when to change the carbon is not quite appropriate since a yellow color means already a far too high pollution level. Furthermore the onset of lights containing more blue and less yellow do not always facilitate the observation of yellowness as in former times.