Anet A6 (may be A8), upgrade Y-Belt (inside).

Y-belt tensioner, hidden type. Does not go outside the printer.
The tension is controlled by rotating the outer body of the tensioner.
The axis screw (M3) was shortened by 2 mm.
You need two screws with a head of 10mm and a length of not more than 15mm, for attaching to a printer.
I use https://www.thingiverse.com/thing:2348612, https://www.thingiverse.com/thing:2196720.