Ah, yeah that kind of belt arrangement. I have that on my OX router, and I pulled pretty hard on both ends (roughly equally tight) when setting it up, then did it again a few weeks later after the belts stretched a little. You’ll probably want to re-square the gantry after setting belt tension; otherwise it might hold a little crooked. (The OX has two motors instead of a torsion rod, which makes the process of squaring the OX different.)
The belt segments that go over the drive pulleys should be definitely tight.
Also check the preload on the eccentric nuts in the gantry plates. Each of the wheels should be possible to turn by hand while the gantry isn’t moving, but shouldn’t turn loosely. The wheels should be fully engaged with the rail, but not wearing out. If they are too tight and you can’t turn them with a finger, you are deforming them and there is too much drag and the belts are stretching in alternating directions. If they are too loose, you will have backlash from the gantry rocking slightly on direction changes.
I hate to be that guy, but…
I’m hoping that one of the plastic-wrapped items in the kit was safety goggles, and that you know whether they are good; some chinese kits have had bad goggles. In general, running a high-power laser like this without an enclosure is an extremely bad idea. Blue lasers are more damaging to eyes than more powerful CO2 lasers because they reach your retina directly; one specular reflection around the side of your goggles and that’s a part of your — or your wife’s, or your child’s — retina that will never grow back. CO2 lasers will more likely “only” destroy your cornea and then iris as they relatively slowly burn out the vitreous humour, so you might regain some eyesight after you have a cornea transplant from a cadaver. No such luck with blue lasers.
There is a catch-phrase in the firearms enthusiast community: “gun safety is no accident.” The same is true of laser safety.