When turning tank lights on and off, we must bear in mind the needs of the fish. We need to remember that fish's eyes do not react to light in the same way as our own. And we need to remember that fish are 'programmed' by nature to expect certain things in their environment.
Fish have eyes which are similar to ours. We both have a cornea, iris, retina etc. We both have rod cells in the retina which allow us to see in dim light. We both have cone cells in the retina which allow us to see colour. The main difference between us and fish is the way we control the amount of light getting into the eye.
We have eyelids we can close; except for a very tiny number of fish, they have no eyelids.
Our pupils can get bigger or smaller to allow more or less light into the eye, fish's pupils cannot change.
Fish's rod cells can sink into the retina in bright light and the cones can come nearer the surface in dim light. Our pupils can change size very quickly but it takes about 30 minutes for the cells in the fish's retina to move.
Fish cannot adapt quickly to changes in the amount of light. We should not do anything to the tank for at least 30 mintues after the tank lights come on – we should not feed the fish, do a water change, rearrange decor etc during this time.
And we should not turn the tank lights on or off in a dark room. Doing this stresses the fish, and we know that stressed fish have a lower imune system so they get sick more easily.
The room should have some lighting, either daylight or the room's light, for at least half an hour before the tank lights come on, and at least half an hour after the tank lights turn off. An hour is even better.
Humans and fish, like most animals, have a circadian rhythm or "internal body clock". Upsetting this circadian rhythm causes an imbalance which has to be reset – this is the cause of jet lag. The circadian rhythm is controlled by light and dark. In humans, our eyes are the main way we detect light and dark, though many of our other cells have a sensitivity to light. In fish, the sensitivity of the other cells is very high.
It has already been mentioned that the cells in fish's retinas move according to the amount of light. But these cells move in anticipation of changes in light. Fish "know" when dawn and dusk are coming and their eyes start to adjust in readiness. In nature, the time of dawn and dusk is very predictable, especially in the tropics where most of our fish come from. But our tank lights are not predictable to the fish if we just turn them on and off to suit us. We should have the lights turn on and off at the same time every day because the fish's circadian rhythm "expects" this.
Fish also have a need for a period of total darkness in every 24 hours. No tank lights, no room lights on, no daylight. In the regions our fish live in the wild, day and night are just about the same length – 10 to 12 hours total dark, 10 to 12 hours bright light, with dawn and dusk of half light between.
Tank lights do not need to be on for the same duration as this. The number of hours total darkness and bright light can be shorter or longer than in the wild – to the fish this just seems like a longer or shorter dawn and dusk. But they must have some hours of total dark and bright light in every 24 hours, with periods of dimmer light between. These hours of total dark and bright light should be the same length and at the same time of day, every day of the year.