Hi Vito,
It sounds as if you're asking whether adding more nutrients causes a nutrient deficiency. While adding more of one nutrient causes an increased uptake of another, I'd probably avoid that particular connection. Certainly, adding more CO2 creates a demand for more nutrients so if you don't make an adjustment to the dosing level after a CO2 injection increase, it is possible to have a nutrient shortfall. Of course that all depends on lighting, flow and all the usual culprits.
In any case you changed more than one variable, so drawing a correlation between cause and effect is completely invalidated. If you want to test the effects of higher dosing level then you must leave all other variables constant. Light, flow, CO2, water change routine, maintenance practices, feeding and so forth must all be held constant. Then note the effect after three weeks or so. Similarly, to test the effects of each of the other variables, hold the dosing constant and change a single variable.
There is a probability that if you held dosing constant and only increased CO2 some plants can actually decline depending on how close that particular specimen was to the critical CO2/nutrient uptake threshold. So for example, if a given specimen, in a given location and under a given lighting/flow configuration was at its limit of nutrient uptake rate (but not at it's CO2 uptake limit) then, suddenly increasing the CO2 may cause a nutrient uptake shortfall which could easily result in either a deficiency syndrome or even algae. The uninitiated may then easily conclude that CO2 causes algae - which would be a false correlation.
Because of the complexity of the interactions, and because we have difficulty holding variables constant (or even knowing what they are) changing multiple variables in an environment is not a good procedure for determining cause and effect, and any conclusions drawn based on these dynamics would not be logical.
If you want to confirm which variable caused your hairgrass to turn yellow, you'd have to go back to the beginning, control each variable as much as possible and only change one at a time. Most don't have the patience required to do that. :?
Cheers,
