A sky themed tank sounds interesting but also hard with all the options.
Do you want the deep clear blue of a warm summers day without a cloud in sight?, therefore perhaps all blue coloured fish. Or even mostly blue coloured fish with perhaps a couple of white fish to represent clouds (kissing gouramis spring to mind for this job).
Do you want a storm rolling in so some darker more energetic fish that are always zooming about like lightening? Something like barbs or danios spring to mind for this task.
Or a picture perfect sunset with pinks, purples, oranges and yellows all awash across the sky.
Or even a rainbow? Which could also have schools of fish that are the colour of the rainbow.
Once you know what type of sky you want, then you need to track down the colours or fish habits that reflect the desired sky. And then the frustrating part of working out what will be available, compatible and that you like the look of.
Normally I always go for natural looking tanks with timbers and live plants, but for a sky themed tank I don't think you could go to wrong with sneaking a TARDIS into the tank. Nothing screams sky, space, new places more than the good Dr Who's TARDIS.
Best of luck and keep us posted on your progress.