hi gang. If I am allowing the CO2 into a cannister filter do I need to be concerned about this @ nighttime ? I have a 75 gallon planted tank running a 2217 E-heim cannister filter. Other than disconnecting the hose manually when the lights go out what are my choices?
The short answer is..."it depends"
If you have an extremely heavy plant load, the amount of CO2 they make at night could push it over the limit for fish (depends on their sensitivity, etc).
However, this problem is more common with pressurized CO2 and not DIY CO2. You did not mention how many bottles of DIY CO2 you are running, but it is doubtful that it will generate enough CO2 at night.
If you are concerned, you could put an air pump with airstone on a timer so that it comes on at night, but again, unless you are running a large number of bottles on your 75 gallon aquarium, you should be fine.