New w124 owner need someone to explain the tensioner to me.

[hanly2]

Here is some background. 1990 300e, m103. I noticed the belt was loose so I adjusted the tensioner the belt tightened back up, but I was not sure if I tightened the 19mm bolt tight enough (73lbs did not even seat the washer) so I snugged it up a little more. 2 days later and the belt seems loose again. So I would like to understand it some more.
1. If the tensioner is manual adjustment how does it go bad besides a squeaky pulley?
2. If I grab the belt and move it should the tensioner move or be stable?
3. I tightened until the arrow moved to the left side, it is still on the left side even though the belt feels looser.
Thanks.