OK to turn off CO2 earlier?

Depending on your photoperiod I'll say. If you have a photo period of 12h, then 4 hours without CO2 could be too much. The question is: why would you give them light for 12h and why would you let them some hours in a lack of an important nutrient: CO2

 

I believe it is quite common to do what you described above. Turning on co2 1 hour before the lights come on gets your co2 levels up so that when lights come on your plants should have access to what they need. I don't see any need to run co2 at night which may stress fish. I think concerns about co2 stability if it is turned off have been over stated. I also don't believe that fluctuations in ph is of great concern either. I have found that almost all fish can adapt to even significant changes in ph. I believe Tom in a past post referred to a study of lakes where ph swung several points as a result of runoff after a rainfall with no affect on fish. I think high co2 levels is a greater threat to fish.

Henry

 

His question isn't about CO2 at night, but to turn it off after 8h. I think it can only be answered based on photoperiod. 1-2h before lights are off can be ok, depending on the setup

 

Scott,

As long as the lights go off when the c02 does I agree that the c02 run time PRIOR to lights on is also vital. My c02 goes on about 90 minutes before lights on.

I would agree with Jonny that about 60-90 minutes would be the max I would be comfortable turning off the c02 while leaving the lights on esp at the same rate/intensity.

If you reduce the intensity for this time when c02 is off, it is another story, but I still think it is not a good idea. I think that c02 does not stay in solution long, so the amount in the tank will quickly be used, and then a deficiency exists.

We have learned the importance of c02 and how changing levels affect the amount of Rubisco produced by the plants. Why take the chance that these may be affected for no apparent purpose?

I would not recommend leaving your lights on for 4 hrs with n0 c02.