First and very importantly...tank lights must be on for one continuous period during each 24-hour period. The tank lighting period is "day" to the fish and plants and it needs to be a continuous period whatever the duration (another issue I'll come to), so use a timer. The "day" can be whatever period you want, obviously best when you are normally home to enjoy the aquarium. There must be ambient light in the room (daylight or artificial light) just before the tank light comes on, and for a period after it goes off. Roughly an hour is best, or more depending upon your "day" period. There must be several continuous hours of total darkness with no room lighting, in order for fish and plants to "rest." Lighting is extremely crucial to fish especially.
The duration of the "day" can be six hours or more. You want to find the duration that will not encourage algae. This is the light/nutrient balance wherein plants use the light and algae is disadvantaged. I have my tank lighting on for 8 hours each day, with one tank at 7 hours (algae begins increasing with 8 hours in this tank as the lighting is a bit more intense). Six hours is minimum, and beyond that whatever works in your situation.
