we are more likely to observe CO2 related faults in a CO2 supplemented tank, as counter intuitive as this may seem...
It is counterintuitive, indeed, because CO2 fluctuations in
relative terms are much stronger in low-tech, and even more so in places with natural illumination.
Both illumination and CO2 concentration vary considerably, abruptly, and often unpredictably in natural habitats. Therefore I rather disagree with often mentioned argument that "plants need time to adjust" to these variables. Of course, plants re-create many proteins (normally about 15 to 25 % per day) to reflect their changing needs and priorities. However, they do not have to adjust to variables which are - within certain limits - changing all the time.
It is quite normal that CO2 concentration fluctuates between zero to about 5 mg/L every day. It is normal that plants uptake CO2 in the morning and bicarbonates in the afternoon. At the same time, every day is quite different in sunlight/cloudiness, temperature, and surface agitation (winds, rains). There is nothing like stability in natural habitats. If plants needed "days" to adjust, they would die first.
Compare that with a high-tech tank with regulated temperature, constant illumination, zero winds/rains, and - what a disaster! - CO2 concentration falling from 30 to 15 mg/L, i.e. mere 50 %, and still in abundance.
I respect observations made multiple times by many people. But if I try to comprehend it, I find suggested explanations somehow unsatisfactory.
In a CO2 injected tank the issues are more likely to be CO2 related. Conversely in a non injected tank much less likely to be CO2 related, but doesn't mean it isn't CO2
Correct, but then...
I have been adding Tropica Premium fertiliser, plus doing a 50% weekly water change. Overall the health of the plant leaves has improved.
... we see that other causes are "possibly possible".
Carbon builds plant structure, so any failure of structure can only mean a failure in Carbon.
This is a simplification which should not pass unnoticed. It is true that structural tissues are generally poor in nutrients and are formed primarily by polysacharides, i.e. (CH2O)n. But structural defects caused by nutrient deficiency are well described for calcium, boron, copper, and other nutrients.
Secondly, there is only indirect link between photosynthesis and structural tissues building. Plants are permanently full of simple sacharides which are continuously distributed (through xylem & phloem) to all plant's organs. Troubles are more likely caused by missing or malfunctioning
enzymes, whose central atom is often a micronutrient.
I can speculate of many possible effects of varying concentration of CO2 on various enzymes, but I'd never call any of them "a failure in Carbon".
When it comes to "pinholes" in leaves, my bet is neither carbon nor potassium, but rather magnesium. Not necessarily an absolute lack of Mg. It may very well be functionality of some enzymes responsible for chlorophyll creation/maintenance which is compromised locally by something (pH, H2O2, ...) which in turn is caused by CO2 fluctuations. I don't know of any scientific paper which explains these observations.