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Issue: February 2007

Aquarium Lighting

Author: Laura Muha

SF 0207
Photographer: Iggy Tavares
The Skeptical Fishkeeper: February 2007

I live in the country, but a couple of days a week I commute into the city for my job. I take an early train, and that means that at this time of year it’s still dark when I stagger downstairs for my first cup of coffee.

I know, I know—this is supposed to be a column about fish, not commuter woes. But bear with me, because the two are more closely linked than they might seem.

You see, like many people whose fish habit has gotten a bit out of control, I have tanks everywhere—the hall, the family room, my home office. And to get out of the house in the morning, I have to walk past almost all of them.

This wouldn’t be a big deal had I not been told repeatedly that sudden changes of light are stressful to fish. In nature, after all, they don’t go from daylight to darkness (or vice versa) with the flip of a switch; rather, the lighting changes gradually as the sun rises and sets. So, the maxim goes, you shouldn’t turn off the tank lights and room lights simultaneously at night, because the sudden plunge into darkness will upset the fish. Presumably, the reverse is also true; if it’s pitch dark, suddenly turning on the lights will also stress them out. 

This complicates my morning routine, because a long hallway leads from our stairs to the kitchen, and my marine tank sits in the middle of it. I’m afraid turning on the overhead light would panic the fish, so I proceed gingerly toward the kitchen, straining to see by the dim glow of lamplight filtering from the second-floor landing. When I get to the kitchen, I adjust the dimmer switch downward before turning on the sink lights, to avoid startling the fish in the adjacent family room. Then I pour some coffee and look around for my briefcase. Darn! I must have left it in my home office where—you guessed it—there are more tanks. As I stumble around in the dark, I trip over a toy one of the dogs has left on the rug and utter a word that can’t be printed in this magazine.

“This is ridiculous,” says my husband. “Why don’t you just turn on the lights?”

Why indeed? While it makes sense that sudden changes in lighting would be stressful to fish (think of our own startled reactions when a late-night thunderstorm takes out the electricity, or the lights go back on just as our eyes become accustomed to the dark), one thing I’ve learned in my decades writing about science is that not all things that make sense on the surface pan out scientifically. So I decided to investigate.

Effects of Lighting Changes

I already knew that light has many well-documented effects on fish. For instance, changes in the light cycle trigger breeding behaviors in some species; its intensity and the length of time that fry are exposed to it can affect their growth. Development of pigmentation in fish is also partly a function of light.

But none of that really had much bearing on the question I was trying to answer, which was whether sudden changes in ambient light could impact the overall well-being of fish. To get at that, I thought, it might help to understand how a fish’s eye reacts to light. Consulting an ichthyology textbook, I discovered that the eyes of most fish are not all that differently structured than our own. Like us, fish have a cornea, a lens, and a retina. And their retina, like ours, is packed with specialized cells called rods and cones that enable them to see in low light (rods) and perceive color (cones). These cells are quite delicate, so in humans, the amount of light reaching them is controlled by the pupil, which expands and contracts like the aperture of a camera as the light brightens or dims. We humans also have the ability to protect our eyes by squinting or blinking if the light becomes too intense.

But with few exceptions, fish don’t have eyelids, nor can they blink. And the pupils of most species don’t expand or contract in response to light the way that ours do. Rather, when exposed to bright light, the rod cells of most fish react by migrating deeper into the retina, while the cones move toward the surface. When the light dims, the rods move toward the surface while the cones retreat. But this process takes much longer than the almost-instantaneous expansion and contraction of the human pupil. Studies have shown, for instance, that it takes salmon about a half hour to adapt to bright light, and an hour to adapt to low light. And studies suggest circadian rhythms also affect how quickly the process occurs. In other words, under normal circumstances, the process may not be triggered entirely by changes in lighting, but may be somewhat pre-programmed to anticipate them. So the amount of time it takes a fish to adjust to a change in ambient light could vary according to the time of day and the corresponding spot in its circadian cycle.

Digging deeper, I found a few references in the scientific literature to the impact of sudden changes in lighting on fish. For instance, a 1977 study of larval sailfin sculpins Nautichthys oculofasciatus found that survival was lower when the larvae were exposed to abrupt changes in light (although another study on walleyes found no relationship between light and survival rates).

In addition, a 1998 article on aquarium lighting in The Progressive Fish Culturist, a journal published by the American Fisheries Society, included a summary of scientific literature on the subject. It reported that when the lights went on suddenly, some fish sank or dove to the bottom; dashed around frantically, sometimes bumping into objects or jumping out of the aquarium; or seemed to fade in color. Under similar circumstances, goldfish sometimes show altered breathing and heart rates, believed to be a stress reaction. And Jon Hoech, curator of fishes at the Monterrey Bay Aquarium, said he’s seen the same thing when the lights are turned out suddenly. For that reason, he said in an e-mail, “We do slowly ramp down the lighting in many of our exhibits. For example, both [the] Monterey Bay Habitats Exhibit and [the] Outer Bay Exhibit are darkened at night by sequentially turning off rows of lights at a time, rather than the whole lot at once. … In our smaller exhibits, we rely on external lighting that typically always stays on longer than exhibit lighting in such a way that there is a more gradual change from ‘day’ to ‘night.’”

Part of the problem is that aquarium fish experience what Thom Demas, curator of fishes at the Tennessee Aquarium in Chattanooga, calls “the kitchen-window effect.” That means that how much captive fish see of their environment depends on the contrast in light levels inside and outside of their tank. “It’s like standing at your kitchen sink at night looking out of the window,” Demas explained. “You can see little [of what lies beyond it] unless the outside lights are on.” Alternatively, if the kitchen light is off but the patio lights are on, you’ll see the patio a lot more clearly than the room you’re standing in.

That means that if I come downstairs in the morning and flip on the lights, my fish will “wake up” (see sidebar, “Do Fish Sleep?”) to find a large, well-illuminated moving object—me—coming toward them from the world beyond their tank. Even putting aside the question of what I look like first thing in the morning (something that presumably bothers me more than it does the fish), I’d be pretty rattled if I were them. If they attempted to flee, however, they wouldn’t be able to see where they where going in the darkened tank, and they’d be at risk of bumping into something and injuring themselves. And even if they didn’t, they’d be likely to be quite disoriented.

“So what?” asked my husband, who has never been as solicitous of the fish as I am.

“It would stress them out,” I said, glaring at him.

He replied, “I repeat: So what?”

The Impact of Stress

That brings us to the impact of stress on fish, a subject complicated enough to have merited a column of its own (TFH December 2005). However, since it’s relevant here, I’ll recap briefly. Stress, as Demas explains, is essentially 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 less it with less energy to deal with other things, such as pathogens. The implications are obvious: “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,” Demas explained.

Some species are notorious for being stressed out by changes in light. Public aquariums, for instance, not only use nightlights in their sea dragon tanks, but keep those nightlights on backup generators in case of power outages because the dragons get so upset when the lights go out that they shoot to the top of their tanks and begin to “spy hop”—that is, bob up and down at the surface, often ingesting air in the process. “Also, when the lights come up in the morning, without a night light they get very excited and swim about seemingly out of control; clearly a deviant behavior,” says Demas. 

What’s The Solution?

The good news is that many skittish fish can become desensitized if they’re exposed to a stressor—in this case a sudden change in light—often enough to realize that it isn’t dangerous. And many other species of fish don’t seem to be all that upset by it to begin with. “Lots of fish do just fine with the lights popping on or off regardless of the light level of the room they are in,” said Demas. “This may not be in their best interest—who really knows?—but they do seem to handle it.”

Public aquariums usually try and mimic the normal light cycles that fish would be exposed to in the wild. “Sometimes that includes playing games with the way we lower lights, raise lights and the duration of light, as well as simulating moonlight,” said Marc Kind, curator of fishes and invertebrates at the Adventure Aquarium in Camden, New Jersey. In some galleries, the room lights are left on all night. In others, they’re raised and lowered slowly.

Added Demas, “We do have systems that have computer control to simulate sunrise and sunset, but I have not kept a fish that I felt it was absolutely necessary to simulate to that extent. Usually using multiple lights and turning them on or off one at a time and possibly incorporating a night light has always worked for us, even in the most extreme cases.”

Of course, most hobbyists can’t afford fancy electronically controlled lighting systems, and fortunately, few of us need them. “Making a decision on how to manage your lighting should most likely be based on the sensitivity to light by the species you keep and weighed against practicality and cost,” Demas advised.

Hobbyists in my situation (those who get up early or come home late) for example, could use automatic timers to turn room lights on or off one at a time, exposing fish to changes in light gradually. Or they could install an actinic bulb on the tank and set it to go on first and off last; because actinic lights are blue, they tend to be less startling to fish. Alternatively, they could do as I do, turning the lights up gradually by hand to avoid catching the fish off guard.

And that, Kind said, is just good, sound husbandry.


Do fish sleep? “I think it’s pretty much the consensus that they go into some slowdown mode or rest period where they either hover, wedge themselves somewhere or swim in a very consistent pattern like we see with sharks,” says Marc Kind, curator of fishes and invertebrates at Adventure Aquarium in Camden, New Jersey. For diurnal species, this tends to occur when it’s dark; for nocturnal species it usually occurs during the day. However, Kind added, it’s not clear whether this resting state is actually “sleep,” because in order to meet the scientific definition of that state, a resting fish would have to exhibit certain patterns of brain waves, and there’s no way to hook it up to electrodes in a sleep lab to see whether it does.

However, it’s clear from the experiences of scuba enthusiasts that fish often need a moment to recover when disturbed while in this resting state. Take parrotfish, which surround themselves in slime cocoons at night, making it obvious when they are at rest. “Since they can’t close their eyes or regulate pupil size, parrots are often awakened and confused by divers’ lights at night on the coral reef,” says Steve Webster, the now-retired senior biologist and co-founder of the Monterey Bay Aquarium in Monterey, California.

>>>Sidebar 2<<<

Shutterbugs beware: bright camera flashes can also be stressful to fish. “When we work with professional photographers (who typically have industrial-strength strobes) we have to be very careful about strobe levels, as too much light in this fashion can result in fish becoming disoriented and running into each other and exhibit walls,” says Jon Hoech, curator of fishes at the Monterey Bay Aquarium.

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