# Lighting: How It Affects Freshwater Fish



## Byron

*Part One*

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.

_[continued in Part Two]_


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## Byron

*Part Two*

*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.

*References:*

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:
http://www.advancedaquarist.com/2007/1/aafeature2

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


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## MyLittlePleco

Very interesting article. I would never have thought that lighting could affect fish in such a way, although it seems like even the shyest of fish can get used to high light environments (Discus, for example).

Thanks for the lesson.:-D


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## Byron

MyLittlePleco said:


> Very interesting article. I would never have thought that lighting could affect fish in such a way, although it seems like even the shyest of fish can get used to high light environments (Discus, for example).
> 
> Thanks for the lesson.:-D


Actually, they never get "used" to it, which word implies some sort of acceptable non-issue status. They "tolerate" it because they have no choice, unless we give them some shade. The stress of bright light will have a negative impact, that is well known, but exactly what can vary.


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## MyLittlePleco

Byron said:


> Actually, they never get "used" to it, which word implies some sort of acceptable non-issue status. They "tolerate" it because they have no choice, unless we give them some shade. The stress of bright light will have a negative impact, that is well known, but exactly what can vary.


I see. I went ahead and assumed that fish will adapt to bright light with no significant side-effects. From what I can tell, it seems like the only way to keep stress-free fish and healthy high-light plants in the same tank would be to provide a spotlight effect, so that the fish have the option of remaining in the shade, or maintain enough cover in a lit-up tank for the fish.


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## bigfish93

I always loved the look of the high tech tanks with tons of light, CO2, and nutrients with tons of red and green plants. I was going to go this route until Byron informed me that these tanks are not suitable for fish. That is what made me want to go the natural route and I have not regreted it. I am so glad Byron told me about this otherwise I would have some very stressed out fish from bright light.


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## Byron

MyLittlePleco said:


> I see. I went ahead and assumed that fish will adapt to bright light with no significant side-effects. From what I can tell, it seems like the only way to keep stress-free fish and healthy high-light plants in the same tank would be to provide a spotlight effect, so that the fish have the option of remaining in the shade, or maintain enough cover in a lit-up tank for the fish.


Within reasonable limits, it is quite feasible to have sufficient light for plants with forest fish. Floating plants obviously do a tremendous job in this; I can assure anyone that all forest fish appreciate floating plants over them, it calms them remarkably. Another trick is thick plant growth or plants with nice wide leaves. Similarly, having bogwood, perhaps rock crevices. Catfish and loaches frequent crevices and caves, and many do not realize this is due to the light. Catfish have very large eyes per body size, because they live in very dim murky water; many are nocturnal. The Loricarid catfish are one of the few groups that do have adjustable eyes to shade out light, and several of these live in brighter creeks; they have a papillary iris flap that expands in strong light and contracts in less light.

Byron.


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## bigfish93

Here is an example of a high light tank with some very stressed out fish because they have no where to go for shade.


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## Byron

Good example. I truly feel sorry for those neons. They are washed out, undoubtedly due solely to the light. Swimming back and forth within a small area (not using the entire tank) and huddled together in a tight group is clear evidence of stressed fish. And with all those plants one would expect the water to be good, so there is nothing else to explain such abnormal behaviours.

By contrast, here are some fish in their natural habitat in Amazonia. Happen to be the cardinal, but still indicative of the habitat.


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## Santaclaws

I would just like to add you can also add a dimmer switch to your light and turn it on and off gradually this way its more like the sun coming up and going down I also believe this aids with breeding.


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## BarbH

Byron thanks for another great article :-D


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## CinBos

Santaclaws said:


> I would just like to add you can also add a dimmer switch to your light and turn it on and off gradually this way its more like the sun coming up and going down I also believe this aids with breeding.


How could you incorporate the dimmer switch to mimic the sun rise and sun set without manually do so?


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## Hallyx

You can't use a dimmer with florescents.

You can, however, get timed dimmers.


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## Chesh

Glad you posted this - and that I found it! I've been wondering how fish's eyes work - thanks!


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## Pearl2011

Those neon are so . . . bright! They look almost purple when near the surface! So awesome!
Byron, In my ten gallon with female bettas when I turn the light they never look stressed, ever. They dont get frantic or anything, just beg for food they do whenever somebody looks at the tank. Is this normal?


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## Byron

Pearl2011 said:


> Those neon are so . . . bright! They look almost purple when near the surface! So awesome!
> Byron, In my ten gallon with female bettas when I turn the light they never look stressed, ever. They dont get frantic or anything, just beg for food they do whenever somebody looks at the tank. Is this normal?


If there is sufficient ambient light in the room when the tank light comes on or goes off, fish shouldn't be bothered.


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## Olympia

I think it depends on the fish too. My betta never stress, but my cories will go nuts if I turn on the light in a dark room.


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## Chesh

Yes, and tetra really don't like the lights suddenly being switched on, either. I actually try very hard to make the lights in my home kind of turn on and off gently for all of my tanks. . . First I open a window shade, then turn on the overhead lights, before finally turning on the tank lights. Same in reverse at night, only the window-shade is replaced by a dim lamp, or a light in the next room (depending on which tank it is). I don't know if this helps them or not, but I TRY!!!


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## Pearl2011

When I turn it on or off I usually have a light so I can read in the morning, soit is not pitch black


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## Brandonl87

Hey Bryon, I just came across this articale and found it very interesting since i just moved my African Cichlids to a new tank thus a new light also. Which im using a home depot light fixture and just regular flourcent tubes. wht would you recommend for light on a 125g tank? Where i can dupilcate sunset sunrise?


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## Byron

Brandonl87 said:


> Hey Bryon, I just came across this articale and found it very interesting since i just moved my African Cichlids to a new tank thus a new light also. Which im using a home depot light fixture and just regular flourcent tubes. wht would you recommend for light on a 125g tank? Where i can dupilcate sunset sunrise?


Light is critical if there are live plants in the tank, and for this it must be adequate in intensity and spectrum. If there are no live plants, then the light is solely there to illuminate the aquarium and the intensity and spectrum are up to you, though one should never have more light than necessary for the health of the fish.

Sunset/sunrise can only be achieved using true daylight, as through the room windows. Once the room is lit, then the tank light can come on. And the tank light must go off before the room loses daylight. With lighting over the tank, you can have more than one tube and have them on different timers to come close to a similar effect.

Byron.


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## Olympia

A friend has something like that for his discus tank.. He has those blue LED "moonlight" lights I think they're called, and they are timed to turn on in the morning for 1 hour, then the regular light turn on. Then in the evening the blue lights go on for one hour and the regular lights are off. I think that's the closest thing you can do with artificial light.


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## Brandonl87

Ah ok ty . No plants are in the tank because they just tear them up.
_Posted via Mobile Device_


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## CinBos

Hallyx said:


> You can't use a dimmer with florescents.
> 
> You can, however, get timed dimmers.


Where could I fine these? And would TIMED DIMMERS work with florescent bulbs?


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## Hallyx

Fluorescent bulbs operate on a very narrow band of voltage. You cannot dim them; they stay at the same output (approximately) until they go out. 

You can get a timer to turn them on and off at any Home Depot sort of place.

You might get a timed dimmer there for use with incandescent bulbs. Certainly a theatrical supply house. You do the Google.


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## CinBos

Gotchya, already got the lights on timers to somewhat mimic sun rises/sets. Have had this set up for sometime, dont suppose I'll change it now. No googlein on my end, thanks folks.
_Posted via Mobile Device_


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## FuelingFire

Byron, thanks for a wonderful Article, Helped me understand a lot, and i know now that once i do get my fish and start my tank my black out curtains will have to stay open to help with the lights. thanks for a great article.


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## pop

Salutations friends,
When I turn on the aquarium light the fish swim up to the top of the tank not towards the back to hide from the light. I guess this is the *Pavlov* effect I feed the fish when the light is turned on and I leave it on for a couple of hours. Most time the residents of aquarium are bathed in ambient light of the room. I have been in forest and there is not much light under the trees. No live plant soon a deep sand bed will be implemented. 

Pavlov is about conditioned response.
pop


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## 1077

I have a standing floor lamp with three way bulb's(bright,dim,dimmer) that is on timer to come on an hour before tank light's come on.
This way fishes aren't spooked by sudden illumination over the tank(s).
Cory's,pleco's, some tetra',s run for cover as soon as the light's come on over the tank, but usually come out within the hour.


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## slojko

Another good article 

Light-oriented swimming of schooling


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