Thanks for all the information.
I was wondering if, perhaps towards the end of the CO2 bottle, the CO2 concentration was to drop due to "normal air" starting to come out with the mix - requiring the needle valve to have to be set higher.
I get my bottle filled at a home brew shop and I was concerned one day when the guy there wasn't able to completely fill my bottle (because his shop bottle ran out).
I noticed that towards the end of the bottle, I had to crank my CO2 up to a ridiculously high level....and I was therefore wondering if perhaps the last part of my bottle was nothing more than "compressed air"....
Must of been some other factor at play that I overlooked.
So ok; not possible.
6' Planted Tank (72" x 18" x 20") - 4 x 30W T8 Tri-Phosphors - 2 x Eheim 2217 'Classic' canisters
Flourite substrate - Ocean Runner OR-2500 + AM1000 - Tunze Turbelle Nanostream 6045
6.8kg Catalina CO2 - Red Sea Pro regulator - Swagelok B-SS4-A metering valve - Vecton 600 UV