Unless they have changed a lot from the original version, they don't work very well.
There is no delay, but you can not have complete darkness.
I had to put mine next to the orange glow of a power strip in order for there to be enough light to trigger it when the main lights went off.
It sounds strange, but you would think if there was no light it would go into night mode, but there needs to be a little bit.
having it behind your light fixture should be OK if you have some moon lights on the tank at night.
Again unless they changed the design the light for the night mode is on the front, but the photocell sensor plugged into the side of the controller.