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Fish are affected by light in many ways. There are several well-documented studies on spawning in some species being triggered by changes in the day/night cycle, and the hatching of eggs and the growth rate of fry can be impacted significantly depending upon the presence and intensity of light. 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. To understand this, we must know something about the fish’s physiology. 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 aquarist should wait at least 30 minutes after the tank lights come on before feeding or performing a water change or other tank maintenance; this allows the fish to adjust to the light difference.

Turning the Tank Light On/Off

When the tank light suddenly turns on in a dark room, fish will dive to the substrate, dash about frantically often hitting the glass sides of the aquarium, or even jump out of the water. The same reactions occur when the tank lights are suddenly turned out. Aside from any possible physical injury the fish may sustain, these sudden changes in the light cause significant stress to the fish. Bright camera flashes can also be stressful in the same way. So also would any unnatural effect such as strobe lighting.

Thom Demas, curator of fishes at the Tennessee Aquarium, defines stress as anything that threatens to disrupt an organism’s normal physical, mental and/or emotional state. The organism must then expend energy dealing with the stressor, which leaves it with less energy to deal with other things, such as pathogens. “If the fish are busy running from or hiding from that weird phenomenon of ‘instant lights on or off,’ they may be wasting energy to this stressor and eventually get sick from something that is most likely ubiquitous and that they would have tolerated had the stressing event not been there,” says Demas. There is now ample scientific evidence that in fish as in humans, stress at any level has a very negative impact on the immune system because it disrupts the physiological equilibrium of the fish.

The solution with tank lights is obvious: the room should always be reasonably well lit when the tank light comes on and when it goes off. As Marc Kind, curator of fishes and invertebrates at the Adventure Aquarium in Camden, New Jersey, says, “this is just good, sound husbandry.” Given the evidence mentioned previously of the time it takes for fish to adjust, the room should be lit for at least an hour before and after the tank light is turned on or off respectively. From my own experience this all but eliminates any frantic reactions from the fish. They will uniformly and quickly swim toward the room light source (be it light coming in the window or from a lamp) when the tank light goes off, but without frantic crashes and jumping into the tank cover glass which will otherwise occur.

The Day/Night Cycle

Most animals have an internal body clock, called a circadian rhythm, which is modified by the light/dark cycle every 24 hours. This is the explanation for jet-lag in humans when time zones are crossed—our circadian rhythm is unbalanced and has to reset itself, which it does according to periods of light and dark. 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, 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.

The Brightness of the Tank Light

“Daylight” can be vastly different for different species of fish, depending upon their natural habitats. Most aquarium fish are forest fish, living in rivers, streams and creeks, ponds and small lakes, and even ditches and swamps. Some of these watercourses are permanently shaded by the forest canopy or overhanging marginal vegetation; some are filled with branches and sunken bogwood; some watercourses are white-water, full of suspended particulate matter that “soaks up” the light and keeps the water murky and dimly lit; watercourses in full sunlight usually have a thick mat of floating plants, and depending upon the position of the sun in the sky, much of its light may be reflected off the surface. In most areas of the tropics, the rivers and streams rise and flood the surrounding forest for half the year, and it is during this period that the fish move into the even darker forest waters to spawn. For fish living in all of these environments, bright light is something they seldom—and in some cases never—experience, or from which they prefer to retreat given the opportunity. It is no surprise that the fish in the brightly-lit watercourses are almost always found at the edge under overhanging vegetation, branches and outcrops, or floating vegetation.

Programmed by nature over millions of years for such dimly-lit environments, the eyes of a fish are designed to capture the maximum amount of light. This allows the fish to have some degree of vision even in dimly-lit water. But when placed in brighter light, the fish will naturally look for cover in order to escape from that light. Aquarists can readily see this in the aquarium; many forest fish when given the option clearly prefer shaded areas. Baensch & Riehl (1987) called it a “light phobia” in characins. And it impacts the fish’s colouration too.

Fish (and amphibians) possess specialized cells called melanophores that contain hundreds of melanin-filled pigment granules, termed melanosomes. The sole function of these cells is pigment aggregation in the center of the cell or dispersion throughout the cytoplasm, thus altering the shade of colour. The melanophore cells are themselves light sensitive and respond directly to light by melanosome translocations, which is why the fish’s colouration pales during total darkness. Pencilfish in the genus Nannostomus have a diurnal patterning; at night the lateral lines break up into dashes, and the fish pales. This has been noted to occur even in blind fish, which shows that it is not sight that tells the fish it is getting dark, but the increase in darkness probably coupled with the circadian rhythm that sets in motion the natural response of the fish to darkness.

But the fish can also adjust the melanosomes voluntarily in response to environmental stimuli, stress, and social interactions. Fish in the store tank often appear pale, or after being netted and bagged will often be without any colour. Fish kept in brightly-lit tanks will frequently exhibit much less intense colouration. In all these cases, this “washed out” appearance is a natural response caused by high stress. Floating plants are one of the best ways to provide shade, along with using the minimum intensity of light necessary; the reward to the aquarist will be brighter colouration and increased interaction between the fish—and that spells healthier fish.


Baensch, Hans A. and Rudiger Riehl (1987), Aquarium Atlas, First English edition.

Evans, Mark E. (2007), “Fish Eyes,” Advanced Aquarist, Volume VI (January 2007) [online:

Huck, Arden (2002), “How Do Bony Fish Eyes Work?” at [online].

Muha, Laura (2007), “Aquarium Lighting” installment in “The Practical Fishkeeper” column, Tropical Fish Hobbyist, February 2007.

Iga, Tetsuro and Ikuo Takabatake (1983), “Melanophores of Zacco temmincki (teleostei) are light sensitive,” Journal of Experimental Zoology, Volume 227, Issue 1, pp. 9-14.

Byron Hosking
September 25, 2011
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