Is it possible to tighten the belts? Or do you have to install new ones when they get too loose?
Check Thingiverse for GT2 belt tensioners. I like the adjustable style, but there is also simpler slip-on versions.
Figured out that the idler mount is broken as you can see in this photo.
Have downloaded the model and am printing a new one.
To answer your original question, yes, you can tighten the X and Y belts. You do not have to buy new ones. You do not need a tensioner unless that is the option you prefer.
How do you tighten them?
The belts are not a continuous loop, there’s a clamp that locks in each end of the strip of belt. It’s under the table for the Y axis, and behind the extruder mount on the X axis. Use an allen wrench, loosen the clamp, use a needle-nose pliers to grasp the end of the belt and tug it gently tighter, then whilst holding the belt tension with the pliers with one hand, and holding the table or print head stationary with the other hand, use your third hand ( ) to tighten the screws down. Done.