Marine Aquarium Basics, Part 4: Lighting

Author: Philip Hunt

The lighting used over a reef aquarium is critical to the health of the inhabitants, but not all fish and corals need the same type or amount of light. Our reefkeeping expert continues his Marine Aquarium Basics series with tips and tricks on picking the right lighting for your tank.

Lights, Camera, Action!

In a previous part of this series, we talked about water movement. One of the interesting features of that topic is the way in which our knowledge and awareness of its importance increased greatly in the last few years and technology followed rapidly to deliver what our aquariums need.

The story of lighting is different. We have long known what we need from lighting, but only recently has technology really caught up—in the form of LEDs. In the last couple of years, these lamps have had a huge impact on aquarium lighting, but this is not an article about LEDs. Developments in this area are very much ongoing, so while LEDs may represent the future of aquarium lighting, they are by no means the entire present. In this article, we'll look at both the lighting needs of marine aquariums and how to meet them with currently available technology.

Let There be Light, But What Light?

As with other aspects of aquarium husbandry, the best place to start when trying to decide what lighting to use is to look at the needs of your aquarium inhabitants, which reflect their wild habitat. Aquarium hobbyists tend to think that only invertebrates (corals and tridacnid clams) have specific lighting needs, and that fish have no special requirements. This isn’t quite true.

Lighting for Fish

Most marine-aquarium fishes prefer quite bright lighting and may behave nervously or more aggressively in dimly lit tanks. There is a reason for this: In the wild, reef fishes tend to be active either during the day (when the light is usually bright) or at night.

Diurnal Fishes

Diurnal fishes (those fishes that are active during the day) are adapted to avoid diurnal predators, but at night, they hide themselves away and tend to sleep deeply, even having evolved defenses to prevent nocturnal predators from detecting their scent, such as sleeping in mucus cocoons in the case of many parrotfishes and wrasses.

Nocturnal Fishes

Night-shift fishes, in contrast, are adapted to life in the dark and tend to hide during the day. Twilight, the transition between day and night, which is well-represented by a dimly lit aquarium, is a nervous time for diurnal fishes, as nocturnal predators begin to emerge, hence the jittery and sometimes aggressive behavior.


The take-home message from this is that for most fishes, bright lighting is best. There are a few exceptions to this: Some nocturnal fishes are kept in the aquarium (some cardinalfishes for example), and for these species, dim lighting is in order. Some deep-water fishes also prefer subdued light, although for different reasons, namely that light levels are always lower at depth. Many such fishes will adapt to brighter conditions, but this may take time, and providing shaded areas within the aquarium can help the process.

Lighting for Inverts

For corals (and clams), lighting is a more complex and critical topic, but the principle of looking to the wild environment still holds true. Most aquarium corals and clams are found in shallow tropical and subtropical seas, and one of the things that strike visitors from temperate countries on visiting the tropics is the brightness of the sunlight. The sheer intensity of light in the tropics gives us one clue as to what our aquarium lighting needs to be: bright. Things are a little more complex than this, however. Our visitor to the tropics is likely to be experiencing the light intensity at an airport, equivalent to that at the surface of the ocean, not beneath the sea.

Beneath the surface, the light intensity drops, even in the clearest, most pristine sea. The water absorbs light, so the brightness of the light reaching a coral depends on the depth at which it lives. Water turbidity adds an extra complication. While coral reefs grow best in clear water, without much suspended sediment, particular areas of reefs may have lower water clarity. Lagoons would be one example. Also, not all corals live on reefs. Many species can be found in murkier water, for example in seagrass beds, on muddy shores, or even in harbors. In these situations, suspended matter in the water column also reduces the intensity of light—and works in concert with the effects of depth.


Depth and turbidity also have a big impact on the color of light reaching corals. The effect of depth is simple: Different wavelengths of light (corresponding to the colors of the visible spectrum) penetrate water to different extents. Light with longer wavelengths (the red and orange end of the spectrum) is less energetic and is absorbed quickly. Short wavelengths (blue to violet in the visible spectrum) are more energetic and penetrate further. The deeper in the sea you go, the bluer things get.


The effects of turbidity are much more complex, as they depend on just what is suspended in the water. Rotting plant material (as might be encountered in lagoons, on seagrass beds, or around mangroves), phytoplankton, and mineral sediments each have their own effects, absorbing or scattering different wavelengths of light.

It is very difficult to take turbidity into account with aquarium lighting; probably the best we can currently do is to use lower light levels for corals that come from turbid water, although in the future, it might be possible to program LED units to deliver light of whatever color is needed. The effects of depth, however, are much easier to simulate, as we can control both the intensity of lighting and the color temperature, which for our purposes is a way of measuring how blue the light is.

Measuring Color Temperature

Color temperature is measured in degrees Kelvin (K). The higher the color temperature, the bluer the light. Sunlight at noon, unfiltered by water, has a color temperature of around 5500K. As corals are accustomed to living under water (although some species may be exposed at low tide), we tend to use lighting with a higher color temperature to reflect this.

Typically, marine aquarium lighting has color temperatures ranging from 6000K to 14000K. These all look white (somewhat yellowish at the lower end of the range), but the higher color temperatures look distinctly cooler, reflecting the higher blue content of the light. Also available are 20000K lights, and these do have a distinctly bluish appearance. In practice, most corals seem to do well under any of this range of color temperatures, but if you want to create a dedicated deepwater aquarium, 20000K lighting will give the aquarium a nice atmosphere of depth and may be beneficial to some corals from deep reef areas.

The main way of adjusting aquarium lighting to account for depth is through selection of light intensity. In the early days of keeping reef aquariums, it was a struggle to get enough light intensity using the available (mainly fluorescent) lamps. Those days are long gone, however, and today it is just as easy, and just as damaging, to provide too much light as too little.

What this all points to, just as with water movement and many other aquarium parameters, is that it is important to know as much as you can about each of your corals and use this knowledge to select lighting.

Who Needs What?

While many corals are reasonably adaptable to different light levels, some are more demanding, at both ends of the scale. Between the extremes of, say, a lot of brightly-colored Acropora species requiring very high light intensity, and Blastomussa species that need quite low light to thrive, it is possible to group corals by their requirements. What follows is not an exhaustive list, but it gives some indication as to the needs of a range of species.

SPS Corals

Corals that require or prefer very intense light include most small-polyp-stony (SPS) corals, for example Acropora, some Montipora, Porites, Seriatopora, Stylophora, and Pocillopora species among others; also, many faviid corals, including Platygyra and Goniastrea brain corals, and Favia and Favites species. Many zoanthids also prefer very bright light, and while they are very adaptable, Sarcophyton leather corals also thrive under such conditions, as do Tridacna crocea and T. maxima clams. The light intensity these creatures need would be found directly under metal halide or LED lamps and close to the water surface. To provide such intense light with fluorescent lamps demands multiple tubes and efficient reflectors.

Soft Corals

The middle of the lighting range, bright but not super-intense, suits many popular aquarium corals, including many branching and encrusting soft corals such as Sinularia, Cladiella, and Lobophytum species; Xenia, Clavularia, Pachyclavularia, and Anthelia species; and photosynthetic gorgonians such as Pseudoplexaura species. Many large-polyp-stony (LPS) corals, such as Euphyllia species, Lobophyllia, Cynarina, Scolymia, Symphyllia, Caulastrea, Trachyphyllia, Catalaphyllia, and Acanthastrea also prefer these conditions. Many mushroom anemones also do well under these conditions, although some prefer less bright lighting.


Among clams, Tridacna derasa and T. squamosa do well under these light levels. Fluorescent lamps can easily achieve this type of lighting, and in tanks with powerful LED or halide fittings, much of the tank area (except directly under the lamps) should be suitable.

Bubble Corals and Mushroom Anemones

At the lower end of the lighting range, bubble corals (Plerogyra and Physogyra species) do well, as do Blastomussa, Duncanopsammia, and many mushroom anemones. For these corals, shady areas in otherwise brightly lit tanks are ideal, or aquariums can be designed around these species, with less powerful lamps used.

Getting the Blues

Blue LEDs and actinic fluorescent tubes, which have a very narrow output spectrum, are frequently used in reef aquariums. The blue light from these lamps causes a variety of coral pigments to fluoresce, which has an obvious impact on the appearance of the aquarium. It is also believed that blue light may affect the production of pigments, at least in some species, although it is also clear that boosting the blue content of aquarium lighting by adding these essentially monochromatic lamps is not essential for good growth or coloration of corals.

The use of actinic fluorescent tubes began many years ago in an attempt to boost the blue content of the lighting available from the various mixtures of metal halide and fluorescent lamps used at the time. More recently, the same approach was used in early LED aquarium lights, for similar reasons. The habit has largely persisted for aesthetic reasons, but as this approach seems to be, at worst, harmless and can look very interesting, particularly when blue lighting is used for dawn and dusk transitions, before and after the main lamps come on, it's not surprising that it remains popular.

Night and Daylight

Along with the intensity and color of lighting, the length of the photoperiod—the time that the lights are actually on—needs to be considered carefully. As our aquarium inhabitants come from the tropics, they are adapted to the day length in their home regions, which doesn't vary much over the course of the year, ranging from roughly 11 to 13 hours of daylight. For most of us living at higher latitudes, this is shorter than our summer days and longer than our winter ones. Does this matter? It is clear that corals need some time in darkness—illuminate them continuously (easily done by malfunctioning timers or forgetful keepers) and they soon look sick. This may be due to damage caused by excessive oxygen production by zooxanthellae.

A general recommendation is to aim for something close to a tropical day length of around 12 hours. This, however, doesn't always work well in terms of having the aquarium fully lit when you’re at home to see it. Shifting the photoperiod to suit your schedule is entirely possible (the ambient light in most rooms is not enough for significant coral photosynthesis), but it can cause problems with fishes. This approach will leave the aquarium in what amounts to twilight for a significant part of the day, and, as noted, fish may behave erratically or aggressively as a result.

Having a longer photoperiod, coinciding more with our 16-hour idea of a day, can be associated with nuisance algae problems, especially if full lighting is on for the whole period. One solution is to step the light intensity up and down during the course of the day, which is most easily achieved with programmable lamps. This should prevent over-illumination of corals, keep your fishes from spending too much time in the twilight zone, and keep your aquarium looking its best when you are there to enjoy it.

Up and Down

It's useful to be able to vary the intensity of lighting when introducing new corals to the aquarium or replacing lamps. In both cases, the intention is to avoid exposing corals to sudden increases in light intensity, which can be damaging. Where it isn't possible to decrease light intensity under these circumstances, pendant lamps should be raised further from the water surface, then gradually lowered over a few days.

Alternatively, you can put a layer of eggcrate light diffuser between the lamps and the water, at first leaving it there all day, then gradually reducing the time it is present. Also, you can reduce the photoperiod, or at least the period of maximum light intensity, and then gradually return it to normal. Finally, when adding new corals, always place them first at the bottom of the tank or in a shaded spot and gradually move them to their final positions.

Getting What You Need: Types of Lamps and How to Use Them

Having established what we’re trying to achieve with our aquarium lighting, let’s take a look at the available choices of lamps. Before we start to look at different lamp types, however, it is worth asking a simple question: What are the characteristics of an ideal aquarium lamp?

First, it must deliver the light that we require in terms of intensity and color temperature. Next, it would be good to be able to control the light intensity in some way. Efficiency is also important; we want the most light for the least power consumption. As another aspect of efficiency, lamps should run cool to avoid the problem of heat transfer to the aquarium water. Finally, we want our lighting to make the aquarium look good—different lamps give different visual effects. With all this in mind, let’s look at the three most common types of lighting used for marine aquariums.

Metal Halide Lighting

Five years ago, metal halide lamps represented the state of the art in reef aquarium lighting. Available in a wide range of color temperatures (from 6500K right up to 20000K) and wattages (typically 75, 150, 250, 400 and, for really big tanks, 1,000 watts), with fixtures available to suit a wide range of aquarium sizes, from single units for small tanks to triple units that can cover 200-gallon systems, these lamps can certainly deliver the lighting that corals and clams need.

The drawbacks of metal halide lighting are that most units aren’t controllable beyond on-off switching, so for dusk-dawn transitions and periods of less than full lighting, they are usually combined (often in a single fixture) with fluorescent tubes; their power consumption is very high; and above all, their heat output can create a lot of problems. The light output from halide bulbs decreases and changes in spectrum over time, so the bulbs must be changed regularly (about once a year), which adds to the high running costs.

Despite these drawbacks, metal halides remain a valuable option, particularly where very bright lighting is needed in large, deep tanks. The visual effect of halides (as point-source lamps) is also very good, creating an effect like natural sunlight, with deep shadows and glitter lines.

Fluorescent Lamps

Fluorescent tubes, like metal halides, have been around for a long time. In recent years, a wide range of fluorescent tubes designed for marine aquarium use have come on the market. This is probably due to fluorescent lighting being increasingly adopted as a more economical and energy-efficient alternative to metal halides. For example, over here in the UK (where energy prices have increased greatly in the last few years), T5 fluorescent lamps are now much more commonly used than halides to light reef tanks.

For most reef aquariums, multiple tubes are required to provide sufficient light intensity, but this is easy to achieve by using a commercial multi-tube fixture or by fitting several tubes into a tank hood—a downside to the latter approach being the spaghetti-like wiring that can result. Using multiple tubes can provide adequate illumination for most aquariums, the exceptions being deep tanks, where the rather diffuse light from fluorescent lamps doesn't penetrate as well as the point-source lighting from metal halides.

While fluorescent lighting is cheaper to both buy and to run than metal halides, this is not its only virtue. It's possible to really fine-tune the lighting by varying the mix of tubes, and being able to switch individual tubes on and off also means that light intensity is easy to vary through the course of the day. Fluorescent lighting also runs cooler than metal halides, although if multiple tubes are used in an enclosed canopy, significant heat buildup can still occur. Fixtures suspended above the aquarium are better in this respect. Fluorescent tubes need regular replacement, as like metal halide bulbs, they gradually lose output, although the process is slower than for halides and changing tubes at 18-month intervals is usually enough.

Fluorescent lighting has a very different visual effect than metal halides, providing very even, maybe rather flat lighting, which looks much less dramatic. This even spread of light means that light intensity is pretty much the same across the whole tank area; whether this is a good thing or not very much depends on the tank design.


The use of LEDs as a practical lighting technology is very recent, but there are already many different aquarium units on the market. The attractions of LED lighting for marine and reef aquarium use are high efficiency, low power consumption, minimal heat transfer to the water, very long life (around 10 years is claimed for most units) with no need for replacement of bulbs, and the potential for sophisticated electronic control of intensity and color.

The market in aquarium LED units is developing fast, reflecting improvements in LEDs themselves and in control circuitry. While early units tended to be suitable only for nano aquariums, with relatively poor light output (compared to metal halides, at least), much more powerful units have since become available, and it's probably fair to say that these units are genuinely equivalent to high-powered metal halides.

So are there downsides to LED units? There is one big one: the purchase price. While lifetime costs are probably lower than for other types of lighting (due to the low power consumption, long life, and lack of need for bulb replacements), the initial investment required (especially when fitting out a large tank) is very high, although prices are gradually falling.

Also, in such a rapidly developing market, it can be difficult to select a unit, especially as units can be very different in intensity, color, controls, and the spread of light they produce. The latter is an important factor; units vary a lot in their configuration of individual LEDs, each of which acts as a point-source of light. Some units use lenses to focus the light into a tight beam; others spread the light over a wide area. Which to choose depends very much on the design of the aquarium. The visual effect of LED lighting, while always tending towards the dramatic, also varies greatly among units.

Before leaving the topic of LEDs, it is worth noting that, at least for less-critical applications, it is possible to use general-purpose units to light aquariums. There is a wide range of LED spotlights, floodlights, and other bulbs with adequate intensity and suitable color temperatures (over 6,000 K), and these can be used for aquarium lighting. These units do not have the sophisticated controls of some aquarium lighting systems but are typically much less expensive and are just as energy-efficient and long-lasting.

Pick What Works for You

As with everything in aquarium design, the choice of lighting is driven by the needs of the inhabitants. That said, there has probably never been a better time to be making that choice, with many options available to meet the needs of a wide range of corals and the ability to do so while keeping running costs down and without heating the aquarium.