"...so it's a question of balance, if the 2 are equal then they would negate themselves thus leaving the Co2 level unchanged over night."
But we know for a fact that the 2 are not equal during dark hours. This is easily tested by taking the pH of the water right before the lights go out, during the middle of the night, and again right before the lights come on. You will see a steady decrease in the pH. This can only come from the increased concentration of CO2 in the water.
There are then 2 possibilities for why/how this happened:
1. The plants cannot use as much CO2 due to the lack of light, so the CO2 rises in the tank since the DIY/pressurized CO2 does not vary in production during the day/night (within reason due to temp changes in the room).
2. The plants are actively using O2 and releasing CO2 as they are breaking down the stored energy. This CO2 is entering the water and increasing the CO2 concentration and lowers the pH.
I think its important to note that in our tanks #1 is primarily responsible for the pH drop that occurs at night, but #2 only increases this pH drop. Either way, it doesn't really matter. All we are interested in doing is lessening the pH swing. This can most easily be accomplished by increasing the agitation in the tank so that the CO2 outgasses faster. I have a HOB, so I increase the flow. Those with canister filters could just as easily turn on a bubble wand at night.
Turning off the CO2 at night will also cause less CO2 by morning, but the problem here is your trading one pH swing for another (and possibly more dangerous). When you increase the agitation but maintain the same amount of CO2, you will have a steady, gradual decrease in pH. When you shut the CO2 off, you will very quickly outgas the CO2 (especially if you have a HOB). This will rapidly swing the pH higher. It will then stay high throughout the night, and then crash back down when the CO2 is turned on (and the crash will be directly related to your light level, and to your method/efficiency of injection). If you have low/medium light and a really efficient injection method (say 1wpg and a powerhead reactor, the pH will drop very rapidly. If you have high light and relatively inefficient injection (I think most of us DIY'ers have pretty basic injection), the pH will drop very slowly since the plants will be able to uptake much more CO2).
I'd rather have a slow, steady drop in pH, than a quick rise in pH, but that's just my opinion.
justin