Planted Tank Obsessed
Join Date: Jul 2012
Location: Los Angeles, CA
I think one can make the argument about fish and plants being on circadian rhythms, so splitting the lighting may not be optimal for the tank. I haven't seen any data to support this however, so it could be just be conjecture. If you think about people and other animals, they get stressed out when you divide up the daylight schedule but in most cases adjust rather quickly. I'm not aware of any long-term studies on humans or other animals about disrupting circadian rhythms.
I used to split my lighting just like you mentioned and I don't think my fish were too happy about it since they seemed pretty stressed overall. If you use CO2 injection, I'd be careful about splitting up your lighting times if your solenoid valve is also on the same time schedule. It takes time for the CO2 levels in your tank to reach steady-state pressure and also for your plants to use up the CO2 in the water. If you don't have enough surface disruption you might accidentally gas your fish in between your morning and evening light cycles.