I did a rerender on all three of them.
I decided I wanted a proper night sky background. But photoshopping that in via masks could only turn out messy since a lot of the objects in the back are blurred out by depth of field. I've learned by now that doesn't work well enough...
So I took one of my sky hdr's and edited them in photoshop to look more like a nightsky and loaded that into blender as and HDRI.
And I like the result way more. I had to get rid of the fog but I preferred having a visible sky instead of a grey background. :)