While I understand the argument about gradual light and dark, I also think there is more involved than just that. In nature many fish fide at night. They hide in roots, under plants and for many bottom feeder in cave type stuctures or other types of hidey holes. The point is that in many cases, the fish have a way to avoid the light.
In my case I only hjave two types of tanks. One is pleco breeding and grow tanks which contain more cover than one might think which serve many more needs than one might think. The rest of my tanks are planted community type set-ups. My problem with my planted tanks is they all become jungles. There are so many places for the fish to avoid bright light or almost any light. If anything. I may not be leaving them enough open space.
So, I think the lights coming on and going off on the planted tanks is not much of an issue as far as the fish are concerned. There is also some ambient light in most rooms where I have tanks. The pleco tank lights are only on when I am working in the tank so I can see what I am doing. I also feed my fish in the evening for the most part. The bottom feeders can find food in the dark, but the open water fish do need to eat during lighted hours.
There are other factors in the lighting issues that I wonder about. The first is lightning during the dark hours. Fish have no way to avoid such sudden bursts of light except to be under cover. So it is not like they are all "light free" during the night. Next, what about nocturnal fish? Would they not need to sleep during the day? How do they fidn dark to avoid light?
Finally. the one thing we all learn sooner or later is that stress can weaken the ability of a fish to fight off diseases and parasites. More over, continued or regular stress over time should have a similar effect. So, I would think that those people who take no action to mitigate the sudden on and off of their tank lighting should have fish more stressed than those who do so. And that should result in the fish of people with on.off systems should have more health issues with their fish in general.
So, in my case I can say I have anumber of fish which were or still are in my tanks for 15 - 20+ years. I have used timers on tank lights since my very first tank before it had live plants, only fake ones.
I would think that the potential harm to fish over time from how lights come on and go off would potentially be the most detrimental in fish only tanks with little or no cover.
Finally, when fish are startled or scared their initial reaction is to bolt for cover or to run away. Normally, they do this to avoid predation, being picked on by the bigger members of their species or because they have wandered into the territory of another species which responds by chasing the intruder off or outright attacjing it. When it comes to predation I have always believed that fish are hard wired to bolt when they see motion that could be a threat. If the fish waits around to be sure it may get their answer from inside the stomach of something. I am not so sure their reaction to lights on, at least, is not partly related to self preservation.
As always, these are just my thoughts on all of this and I could be way off the mark.
