Happy New Year to you as well mate.
Yeah, you should not use the results of one tank to troubleshoot another. We have to address the known causes first and then investigate the other, less common possibilities. There are too many inter-related issues, many of which we lack the control of. Having said that though, to test your first assumption, then keeping the gas 24/7 in that tank would be a good way to check, however, if the injection/diffusion/distribution techniques are fundamentally poor, then you might have only partial success with 24/7 CO2, or, the additional benefit of 24/7 CO2 might appear to work and might just mask the fundamental problems. Another way to verify would be to keep the tank covered with a dark or an opaque fabric for a few weeks so that it blocks ambient light.
The photo I posted is of a tank with the lights off and with the Summer sun setting, perhaps around 8PM. In the morning, the sun is on the far left and not quite as much light comes in from that opposite side prior to gas on. Depending on the plant species, if they are in top health, they should be able to resist the debilitating effects of pre-gas light and there should be some gas left over still in the water from the previous day, especially if the tank is covered. This is an extreme example so it points to one of the basic issues.
Cheers,
