Reef Aquarium Lighting

Lighting for the coral reef aquarium is one of if not THE most hotly debated topic in reef keeping. Lighting is important because it directly affects how we visually enjoy our tanks but more importantly the vast majority of the corals in our tanks are photosynthetic thus making quality light a necessary part of their husbandry. In this article we will first cover some of the basics of the light itself and then discuss the various lighting technologies people use to light their reefs.

Coral and Zooxanthellae

Coral as we know it are a symbiotic relationship between the coral animal and dinoflagellates called zooxanthellae. Zooxanthellae live in the tissue of the coral and contain chlorophyll for photosynthesis. The byproducts of photosynthesis are simple sugars that the coral hosts can use as an energy source. It is for this reason that coral are often described to the lay person as having traits of both plants and animals.

The color of zooxanthellae are varying shades of brown, however there are over 80 known varieties of zooxanthellae. Depending on which types live in the coral, it can give coral a different appearance.

Corals occasionally expel zooxanthellae to control both the amount of dinoflagellates and the varieties residing in its tissue. This is easiest to see in large polyp stony corals where one might see brown stringy material getting spit out from their mouth. This is usually done to adjust for some change in the environment or a stress reaction. The unfortunate bleaching of natural coral reefs is the result of corals expelling nearly all of their zooxanthellae as a last resort to survive their changing environment.

The color of light is important not only for aesthetics but for the photosynthetic process. Chlorophyll does not absorb light uniformly. There are certain spectrums of light that are absorbed and other spectrums that are rejected. The type of chlorophyll most commonly found in corals is Chlorophyll A and it has two major absorption peaks. One peak is at the 440 nm wavelength which is a violet colored light and the other at the 675nm wavelength which is a red colored light.

Light spectrums neighboring those spectral peaks are also absorbed by Chlorophyll A but also by different compounds such as Chlorophyll C and carotenoids.

Color Temperature

The color of light is measured by a Kelvin rating. If you recall from high school physics, Kelvins is basically Celsius but starts at absolute zero rather than the freezing point of water. So zero degrees Kelvin is equivalent to -273.15 degrees Celsius. Imagine a block of matter that we start to heat up. Once you start turning up the temperature, this theoretical black body starts to emit light. It gets glowing hot. The best way to visualize this is to imagine a star. Our sun burns at just under 6,000K and we get yellow light as a result.

Once you get to about 8,000K to 10,000K the black body radiates white light. Between 15,000K to 25,000K the radiation appears blue. So just to clarify, the bulbs in your tank that are 10,000K are not burning at that temperature. You would have a major problem if they were. Your lights are just producing a similar color light as a star that is burning that hot.

Light Intensity

When I started into reef keeping in the early 90’s, the rule was get the brightest lighting possible. End of story. The most I have ever had on a tank was two 400W metal halides, and four 110W VHO fluorescents over a 75 gallon tank. Needless to say it was very bright and very warm.

Fast forward about 30-something years and now I have some corals in extremely dimly lit tanks doing just fine. So in the bright tank example the intensity was probably close to 1200 PAR if not more. In my low light setup, the PAR was about 50. In both cases, coral grew just fine. In fact, with the exception of Acropora the vast majority of the corals we keep here do quite well in low light. What seems to be more important than raw intensity is consistency.

Lighting Technologies

It often gets asked what light is the best light. Unfortunately there is no right answer. First off, there is the difference in aesthetics. Aesthetic choices are purely subjective decisions that only the hobbyist can make for themselves. Most of the time hobbyists gravitate towards the blue end of the spectrum because it is very flattering to corals that fluoresce.

In terms of coral biology, what type of lighting that will work the best for your tank will depend a great deal on what animals you intend to keep. As I mentioned before, corals will adapt to lighting by regulating their zooxanthellae so most coral will grow under any of these lighting technologies. Having said that, it is also entirely possible to have some corals take on a desirable appearance while other corals become less attractive under the same light. Such is the price to be paid for a mixed reef tank. It is often not possible to optimize a light for every coral inhabitant when they come from completely different geographies and natural conditions.

As far as the type of technology to use, it will depend on what aspects of the technology are most important to you. There are also pros and cons to any type of lighting technology whether it is LED, T5, metal halide, or even natural sunlight. Depending on your situation, the drawbacks of one technology might not matter to you. Conversely, the benefits of another technology might not be meaningful to you and get discounted accordingly.

Let’s talk for a bit about each one in turn, starting with the most popular.

Light Emitting Diodes (LED)

The most popular type of lighting fixture in the reef aquarium hobby is LED. LED fixtures are nice because they offer the most in terms of control, are energy efficient, and the LEDs themselves are long-lasting. As for control, some fixtures allow the user to control each LED individually, so the hobbyist can customize the schedule, colors, and intensity. Of all the types of lighting I think LEDs do the best job of showcasing corals by bringing out the most appealing fluorescence. Lastly, the fixtures tend to run cooler than T5 fluorescents or metal halides which is a big deal for hobbyists in warmer climates that have to battle to cool their tanks. It is far more difficult to cool water down than it is to heat it up.

An early criticism of LEDs that that they could not grow or color up corals compared to older technologies. While there is still room for improvement, LED technology has advanced significantly since the early days and many hobbyists have had success using them over their reef tanks. Over the years we have transitioned our coral farm over to LEDs.

There are some notable downsides to LED. First, there is still the argument that some of the most amazing top end growth and color come as the result of using metal halide or fluorescent bulbs. There are still “old heads” that swear by the old technologies’ ability to grow and color up corals and as of the time of this writing, may very well be true.

Second, the electrical efficiency of LED fixtures is more noticeable in the smaller, simpler fixtures such as strip lights, but in most advanced LED fixtures, their electrical consumption is on par with a metal halide bulb minimizing the benefit of electrical efficiency. On our largest aquarium, we are running a dozen LED fixtures that average 200W each. That is 2400W over a single aquarium.

Third, many aquarists have issues with the spotlight nature of LEDs where dark shadow areas on the underside of corals cause the colony to die off. For whatever reason LEDs tend to cast a very harsh shadow. Some companies are developing more ways to spread and diffuse the light coming from their fixtures to create a blanket of light that reduces harsh shadowing, but it still occurs. Hobbyists often have to add additional fixtures not so much for added light intensity, but to combat the shadows.

Also, while the LEDs last a long time, the longevity of the fixture can be substantially shorter than the rated life of the LED because other electronic components fail. The last downside to LED lighting applies to those interested in photography. LED is the worst light ever invented. The color rendition on camera has a very harsh and highly “baked in” color that is difficult to correct. Also, for videography, some lighting fixtures have a noticeable flicker on camera which ruins the footage. It is possible that that flicker may be harming the tank inhabitants as well.

For better or worse, LED are quickly becoming the most popular form of lighting in the hobby if it isn’t already. Compared to the other types it is the newest technology and could see some major improvements down the line that eliminate many of the drawbacks listed.

T5 Fluorescent

T5 fluorescent lighting is a type of high-output fluorescent lighting that uses a narrow tube that is five-eighths of an inch in diameter (hence the name T5). T5 Fluorescent bulbs produce a very robust light spectrum that colors up corals nicely. They typically have a high color rendering index (CRI) which means they can produce a range of colors and provide excellent color rendition especially for photography and videography.

Unlike LEDs, the color temperature of T5 bulbs are not controllable so the hobbyist has to mix and match bulbs of various color temperatures to achieve the desired blend. There are a lot of bulb choices available which can give your tank practically any type of aesthetic. While the color temperature is not controllable, there are some fixtures out there that can dim T5 bulbs to give that dawn/dusk lighting effect (though I don't know if that reduces the life of the bulbs). Lastly, because the light emitted by a T5 bulb is spread out evenly over the length of the bulb, there is almost no trace of shadowing effects that plague point source lighting.

No lighting system is perfect however, and T5 fixtures have their drawbacks. First off, the bulb life is frustratingly short. They begin to decline both in terms of spectrum and intensity right around 9 months and by 12 months, they are a drastically different bulb. Here at the greenhouse I run them a lot longer than that just because they are supplementing the sunlight we receive, but when I do finally get around to swapping out the bulbs, the difference is staggering. The second downside is the energy efficiency while not horrible on a per-bulb basis is not as good as the LED lights.

Despite the low initial cost of a T5 fixture, depending on the number of bulbs in the fixture, it can get expensive to operate. For example, if you have an 8-bulb fixture, the electrical consumption along with the cost of bulb replacements, might make for the most expensive lighting upkeep out of all the technologies. Lastly, the bulbs themselves are fragile and can easily break during shipping, especially when you consider the longer bulb lengths of 48” and 60”.

Metal Halide Lighting

Finally we get to metal halide which I would venture to guess is the least popular form of lighting at the hobbyist level and likely the most popular lighting at the commercial level. The positive aspects of metal halide are its intensity, spectrum, and longevity. When it comes to growing light loving corals, I don’t think any other technology does it as well.

Metal halide bulbs are a point light source like LED’s but are even more concentrated. When shining down into the tank they create a very pleasing shimmering effect which is almost absent under the diffuse light of a T5 fluorescent bulb. Those shimmer lines closely replicate what is seen in nature and there was a study done several years back that indicated corals actually benefitted from them.

The major problem with metal halide is energy consumption and heat. Metal halide bulbs consume a ton of electricity and it will be noticeable on your monthly electric bill if you just installed a new halide fixture. The heat generated by metal halides is also something that had to be dealt with. In large aquariums situated in a large room, some well-placed cooling fans might do the trick. On smaller aquariums or in tight quarters where heat builds up, one might need to install a chiller or increase the air conditioning to compensate. Either way, heat management will further increase the electricity bill.

In terms of controllability, it is practically absent from this lighting technology. They can turn on and off. In some ways, they don’t even do that well all the time because some bulbs require a cooling off period before being able to turn back on. There are dimmable metal halides, but from everything I’ve read it’s not great for the bulbs and some bulbs will shift colors as a result.

Now that we have gone over the types of lighting hopefully you can see that all of them have their pros and cons. What will make for the best light in your tank will depend in large part on the factors that are important to you.

Hybrid Lighting Solutions

Picking what light to go with can be daunting but the obvious solution to deciding on a fixture is to not really decide on anything. You could get a light that incorporates more than one tech. Hybrid lighting systems exist that combine either LED and T5 or metal halide and T5 or metal halide and LED. There might even be some systems out there that is a combination of all three.

That fixture might shockingly expensive so here is a cost saving tip. Let’s say you have some LED fixture you are happy with but want to supplement it with T5, but you don’t like the look of the T5 bulbs. What you could do is purchase an inexpensive T5 fixture with all white bulbs and only run it for about 4-6 hours while you are away from the tank. That way, the corals still benefit from the light and color up nicely but it doesn’t interfere with the aesthetic of the LEDs when you return home.

Natural Light

Tidal Gardens uses a mix of natural light from a greenhouse structure and supplemental light from mainly T5 and LED fixtures. The idea when the greenhouse was constructed back in 2002 was that the sun is the perfect light on the perfect timer and corals grown under this light would be the healthiest specimens for lowest electrical cost.

This premise still might be true, but not necessarily in Ohio here where the greenhouse is located. The difference in lighting from season to season has a drastic affect on the corals and some species don’t like it at all. One might guess that it is the cold dark Ohio winters that are the problem, but it is actually the summers. The winter months it turns out are some of the best for growing just about every coral. If I had to guess it has to do with both the change in light intensity as well as the photo period. In the summer it feels like the sun is in the sky twice as long and that is not a good thing. The shorter photoperiod of the winter months has proven to be a much better situation for the corals here.

Setups that are designed to use natural light would be the most successful in areas that have the most consistent climates, but preferably not in crazy hot climates because it is far more difficult to chill water than it is to heat it up.


Light is a vitally important aspect of the hobby and is a heavily debated topic among reef aquarists. Hopefully this article helps you decide which type of lighting you want for your system. Let us know which you ended up choosing.

Than Thein