I was wondering why prints have very faint vertical lines even when the optical window is absolutely clear of any dust and debris. Turns out the tank film itself has scratches on the upper layer, out of the box:
Circular thing is a reflection of the ceiling light.
It had to be in the frame for the scratches to be visible.
You can see them above and below the reflection.
They span the whole length of the tank, so must be some part of the film extrusion process, or caused by the winding / unwinding mechanism (i assume it comes in rolls) during tank assembly.
Edit: Before someone asks, yes, that’s a freshly unpacked tank, and no, it’s not from the mixer, i put the mixer straight down at the left edge and clicked it into place - each of those lines spans the whole length.