Koen,
Your Moss walls look fanastic. If my tank was a little deeper front to back, I would have to try something like that.
On another note, I would encourage you to shut off your CO2 at night. I resisted using a solenoid for years because, I figured the cost of the solenoid would even out any savings you would have on the gas and hated the idea of adding another timer and power cord to the setup. I did eventually listen to the advice of Tom Barr and others and am glad I did. When I ran CO2 24/7, every time I would adjust the metering valve, I would have to check CO2 levels first thing in the morning and then as soon as I got home from work. With the CO2 being shut off at night, I don’t have to worry about killing anything overnight when CO2 will be high and O2 will be low. I can now feel more “at ease” to adjust the CO2 level a little higher than maybe necessary, since it would be less likely for it to build up to deadly levels during the day when it is being used by the plants. I think you can successful without a solenoid but, I think you might sleep easier at night after making CO2 changes, with it turned off at night.