This takes some explanation. Fish are affected by light in many ways. The health of fish is closely connected to the intensity of the overhead light, various types of light, and sudden changes from dark to light or light to dark. The primary receptor of light is the eye, but other body cells are also sensitive to light. Fish eyes are not much different from those of other vertebrates including humans. Our eyes share a cornea, an iris, a lens, a pupil, and a retina. The latter contains rods which allow us to see in dim light and cones which perceive colours; while mammals (like us) have two types of cones, fish have three—one for each of the colours red, green and blue. These connect to nerve cells which transmit images to the brain, and the optic lobe is the largest part of the fish’s brain.
These cells are very delicate; humans have pupils that expand or contract to alter the amount of light entering the eye and eyelids, both of which help to prevent damage occurring due to bright light. Fish (with very few exceptions such as some shark species) do not have eyelids, and in most species their pupils are fixed and cannot alter. In bright light, the rods retract into the retina and the cones approach the surface; in dim light the opposite occurs. But unlike our pupils that change very quickly, this process in fish takes time. Scientific studies on salmon have shown that it takes half an hour for the eye to adjust to bright light, and an hour to adjust to dim light. This is why the tank lighting must come on and go out when there is good ambient light (daylight or lamps) in the room.
Most animals have an internal body clock, called a circadian rhythm, which is modified by the light/dark cycle every 24 hours. Our eyes play a primary role in this, but many of our body cells have some reaction to light levels. In fish this light sensitivity in their cells is very high. Previously I mentioned that the rods and cones in the eye shift according to the changes in light. This process is also anticipated according to the time of day; the fish “expects” dawn and dusk, and the eyes will automatically begin to adjust accordingly. This is due to the circadian rhythm.
This is one reason why during each 24 hours a regular period of light/dark—ensuring there are several hours of complete darkness—is essential for the fish. In the tropics, day and night is equal for all 365 days a year, with approximately ten to twelve hours each of daylight and complete darkness (there is no moonlight entering the water where most of our fish originate), separated by fairly brief periods of dawn or dusk. The period of daylight produced by direct tank lighting can be shorter; and the period of total darkness can be somewhat shorter or longer—but there must be several hours of complete darkness in the aquarium. The dusk and dawn periods will appear to be stretched out, but that causes no problems for the fish. It is the bright overhead light that is the concern, along with having a suitable period of total darkness. And the "day" period when the tank lights are on should be one continuous period, not sporadic, and it should be the same every 24 hours or it will impact the circadian rhythm causing more stress.
I will assume you have a significant period of complete darkness, meaning no tank lighting and no room light. If the evening lighting is for your viewing, then I would use the white "daylight" lighting. Extending this can cause algae of course, but with a timer you can have the "daylight" at any time of the day you like, such as late afternoon and evening. Followed by the darkness (with at least half an hour of room lighting after the tank lighting goes out), and then the "dawn" period with no tank lighting, and then the "daylight" again.
Byron.
/www.youtube.com/watch?v=ELJosXXudY4
