Could I simply remove a tube and keep it as a spare? But wouldn't doing this open the connection to damp from the tank itself? Maybe I could half twist the tube so it isn't fully connected - this might mean it stays off but I don't know what implications that might have. If I did go down to one would there be any reason to keep the 'day' over 'nature' or vice verse?
Some ballasts will let you disconnect one bulb, while the other continues operating. And some won't, shutting down the other bulb as well. Only way to know for sure is to try it. If it works, the half-twist sounds like a good idea as long as the bulb isn't at risk of falling out, or you could probably find some way to cover the exposed socket. Keep whichever bulb appeals most to your eye, the difference in plant growth will be minimal.
The Wisteria are still growing strong with lots of new growth without black markings. The older leaves are more prone to the edging and as you mention; I am am snipping those away when they become too unsightly.
Excellent. Sounds like you'll be able to get the black stuff under control with trimming and patience.
The Wisteria is actually now beyond the height of the tank and is flopping over the water surface which is acting as a nice shade-maker for my anubias below. Should I let the Wisteria continue to grow at the surface or trim it right back?
No harm in it as long as it doesn't get excessive light at the surface. Typically, that will manifest as GSA on the surface leaves.
The black edges appear on more than one plant type (including my Sword) so it must be part of my tank setup causing the issue?
From my experience with it, the black stuff shows up on slow-growing plants, or parts of plants (like old growth), when the plants are also
under some kind of stress. The extra light, and insufficient carbon for the amount of light, could be the source of stress here. Wait and see what happens when light is reduced.
What I have also noticed is brown patches and tiny holes on some of my leaves which eventually expands and I remove the leaf. This must be a deficiency of some sort. I have light (too much), I have some CO2 and I have ferts. Could it be a potassium deficiency as I do not supplement this nutrient at all. Should I be using Flourish Potassium alongside the Comp Supplement.
Could be a potassium deficiency. Usually the pinholes are surrounded by a bit of yellow leaf tissue, but that varies with the exact plant.
My tanks always seem to need a bit of extra potassium, and for them I consider Flourish Comprehensive, Potassium, and Excel (or equivalents) a bare minimum. However, that may not be the case with your tanks. And when you reduce lighting, it also reduces nutrient requirements; so any deficiency might disappear. If you don't want to accumulate too many different products, you can wait and see with that one too.