Hi,
I appreciate that conceptually, it's often difficult to get ones head wrapped around CO2 instability. This happens because we have a limited understanding of how CO2 is processed and used inside the plant.
It's less a matter of not enough/too much, but rather a matter of gas availability whose mass oscillates above a certain value.
In order to get a better picture, first have a read of the recent thread
http://ukaps.org/forum/viewtopic.php?f=19&t=11320 in which some insights are provided into the chemistry of CO2 usage.
So for a given lighting level the plant requires a specific CO2 level. Lets use some imagination and pretend that we have made a "Contract" with the plant to deliver a specific daily concentration level. We can oscillate slightly around this specific concentration level but at some point, if the CO2 concentration "excursion" is too far either below or above the contract level this causes the plant to decide that it must re-configure Rubisco enzyme production to account for the change in concentration level.
If we were to deliver a concentration level lower than the original contract level - but held that lower level constant, the plant might suffer short term damage but might make the enzyme adjustments. In a couple of weeks it would recover. As long as this new lower level is not acutely low, the only penalty would be slowed growth rate and perhaps a little algae here or there.
If we were to deliver a concentration level higher than the original contract level - but held that higher level constant, the plant would simple adjust the level of Rubisco to account for the higher level and after a few weeks growth rate would simply increase.
Remember, we're talking about a matter of weeks to make the adjustments to new CO2 levels. Each time the concentration excursion breaks the contract, an enzyme adjustment is triggered. Concentration excursions that occur frequently throughout the day causes havoc with enzyme production because a different enzyme level is commanded at each excursion.
On top of that, each time the excursions falls below the original contract level for the lighting, internal damage is caused in a way that can almost be described as radiation poisoning, because high energy electrons (elevated to higher energy due to photon bombardment from the light), which are supposed to be involved in reactions with CO2, have nowhere to go, so they smash into nearby cells causing ionization of the cells molecular structure.
There are a couple of ways concentration excursions can occur:
1. A shoddy regulator may oscillate it's output.
2. Poor flow in the tank could actually affect the concentration levels either locally or unilaterally.
3. Low basic levels and low injection rate might cause the concentration levels to fall during the day simply by the uptake of the plants themselves, especially if you have a high plant mass.
4. So called midday bursts increase the photon bombardment, thus increasing both the uptake and the electron activity. If there is insufficient CO2 for the burst energy level then this counts as a downward excursion.
5. Uncontrolled out-gassing due to filter or other causes such as surface disturbance and so forth can cause levels to drop.
6. Ph Controllers which vary the injection rate to maintain pH cause excursions by default.
These are just the popular ways I can think of. There are probably as many ways as their are the number of hobbyists. Each way by itself may not be too bad, but combinations are additive.
In your case, it may be a matter of increasing the injection rate to allow for the consumption and to help stabilize the level. You may need to look at doing a good trim to lower the biomass. You may need to rework the flow distribution method, perhaps remove large hardscape items, for example. You may need to lower the light or to cut out midday bursts and so forth.
Hope this helps.
Cheers,