I turn off all power to my tanks. The filters I have don’t like it when water level drops too much. Additionally, some fish are more comfortable with dim lighting and lower levels may reduce stress. I have plenty of ambient light to do my work.Hello and welcome
It isn't necessary to turn the light off, leaving it on will make the task easier.
I bought an electric timer for my lights in my first tank and it has been invaluable. There is no way that I would remember to turn the lights on and off at the same time every day. I might not even be home or awake then either. My new lights have a time and ramp up and ramp down built in.I agree with the timer. Fish need the light to be on for the same length of time, at the same time of day every day. Timers make this easy.
The room needs to be in daylight or the room light switched on for at least half an hour before the tank light turns on and half an hour after it turns off. Fish need a dawn and dusk period between tank light on and total darkness, and a room with daylight/room light provides this.
Fish also need a period of total darkness in every 24 hours.
I leave my tank lights on during water changes so I can see what I'm doing.