The main problem discussing and experimenting with LCO2 is that of our collective ignorance. We really do not understand the How and the Why of it's mechanism. We have a fairly well grounded, basic understanding of the photosynthetic processes.
A. We understand that Rubisco's job is to capture CO2 molecules and to deliver the molecules to the Calvin Cycle reaction centers. We know that Rubisco is hugely expensive and consumes a lot of resources to produce and to maintain. In low tech tanks, where the CO2 concentration is low there is a much higher density of Rubisco in the leaf because you need more of the protein to capture the small amounts of CO2. In gas injected tanks, the Rubisco density in the leaf is lower.
B. We also know that during Calvin Cycle, the fixing of Carbon involves some intermediate carbohydrate products until the final product is a type of glucose.
So, for item A. we know that when the plant senses that high concentrations of CO2 is available, it responds by reducing the production of expensive Rubisco. When it senses a lower CO2 concentration it must increase Rubisco production, however because this protein is so complicated and heavy, the increased production requires 2-3 weeks in order to change the density in the leaf to match the new gas concentration level. So it is much easier to reduce production than it is to increase production. When increasing gas injection therefore, it hardly takes any time to see an improvement in health. When lowering the concentration, the plant will suffer because it must now ramp up Rubisco production to account for the loss of CO2 availability.
When increasing the light, the plant must reallocate resources from Rubisco production/maintenance in order to deal with the increased radiation. This may entail new pigment production for protection. When the light is reduced, the plant then reallocates the light gathering proteins and can devote them to Rubisco production/maintenance.
So when we mess around with light and gas, we have some degree of predictability.
But no one except for Tom Barr and Seachem know what LCO2 is and why it works. We do not know if it is actually converted to CO2 and affects Rubisco production, as in item A., or whether it is converted to one of the carbohydrate intermediaries in Item B. and therefore shot circuits Rubisco management.
The advice to carefully adjust LCO2 upwards has mostly to do with not knowing what the toxicity level for various plants and animals are and has very little to do with the mechanism of Rubisco or the adjustment of the leaf to CO2 levels.
You cannot really compare a plant's performance from one tank to another because we have very little measurement data from which to draw any kind of conclusion. Maybe PAR is lower in the smaller tank. Maybe flow over the leaf is better. Maybe gas is dissolved better at that specific location. We have no idea. So best not to draw any conclusion. The sample in the big tank can maybe be moved around to see if performance increases. That's about the best you can do, really. P. helferi is kind of slow to gain momentum but should perk up eventually. On the other hand it might actually be telling you something about the big tank...
About the only other thing you could do to eliminate LCO2 toxicity is to reduce it, observe the helferi and then increase it, or add it to the small tank to see if there are deleterious effects.
It's all stabbing in the dark mate, sorry...
Cheers,
Clive