Over planted tank???
Mostly an old myth… 
While it’s true that plants go through a “respiration” period during the night, the amount of CO2 they release and O2 they consume is minimal compared to the amount of gas exchages taking place in a normally setup aquarium (one with some surface movement). In your case, it is even more impossible to cause such problems in your tank due to the high rate of gas exchanges generated from the wet/dry filter.
However confirming this is simple and therefore best done to be 100% certain. As you know CO2 effects PH and using the KH/PH relationship tables (http://www.gpodio.com/co2_chart.asp) one can easily measure CO2 levels in the tank. Therefore, test KH (once as it does not change overnight) and PH just before the lights turn off at night. This should represent the time where CO2 is at it’s lowest, O2 at it’s highest, and PH at it’s highest due to the lower CO2 levels. The following morning, test PH again to see how much it has dropped. You should see a minor reduction in PH due to the increased CO2 levels, but it is unusual to see a change much greater than 0.2.
For these same reasons, “turning CO2 off at night” is also something not many people do anymore as it actually causes larger PH swings than erogating CO2 24/7.
Let us know what the PH swing is between night and morning.
Hope that helps Giancarlo Podio, LMD
Category: Philippines Internet Koi Society
