You think vermindern is ergative, which it isn't.
There is a class of verb called ergative. These verbs can be used with and without object, and the object in transitive usage has the same role as the subject in
intransitive/unaccusative
usage. An example for this is "decrease": In
The distance decreased.
and
The man decreased the distance.
it is always the distance that decreased, although it is first the subject and then the object. These verbs are very common in English, where this alternation is a common pattern. German also has such verbs, for example "zerbrechen"(to break).
However, in German another pattern is more prevalent. Instead of becoming intransitive, the verbs become reflexive. Having no object would be considered incorrect. We have
Der Abstand verminderte sich.
and
Der Mann verminderte den Abstand.
The passive, as suggested, is also a way to not state the agent:
Der Abstand wurde vermindert.
The meaning is closer to "the distance was decreased". In constrast to "Der Abstand verminderte sich" this explicitly states that there is someone who
decreased the distance (in a most likely conscious action). So "verminderte sich" or "verringerte sich" is the better translation.
Whether a verb is transitive/intransitive or transitive/reflexive has to be remembered. For example, you could also have used to verb "schrumpfen" (to shrink), which is ergative:
Der Abstand schrumpfte.
Note: The terms transitive, intransitive, ergative, etc. seem to be not used consistently. This answer uses definitions from the English Wikipedia.
See also this question about ändern (to change).
