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Big Questions for Coral Lighting: I'd like to mention here now that studies suggest that feeding a
coral in less-than-optimal light, might significantly help the coral
"make up" for what it is not gaining from utilization of
light. Simply explained, corals in lower light will not produce (fix
carbon) as much as corals in stronger light. Feeding the coral can give
it a source of fixed carbon to supplement its reduced production. Since
all stony corals have the potential to adapt in these ways, just about
all stony corals can live under a wide range of lighting conditions.
Interestingly, it's even thought that corals have a mix of
different types of zooxanthellae and can change the proportions of
different types to adapt to different types/levels of light. This
might explain, at least in part, why corals appear to change color
under different lighting conditions. [Note: For more on the photo
adaptation of corals:
Photosynthesis and Photoadaptation by Sanjay Joshi]
I've personally seen Acropora sp. corals kept well under VHO
lighting and Faviids kept well under intense MH lighting. Again, this
begs the question "how the heck do we figure out what lighting to
give a particular coral?" [Note: When purchasing corals online, ask to see recent pictures of the individual coral you will be sold... call and ask the same questions you would of any in-person vendor.] Once your coral has had time to adapt to your system, you can *slowly* move it to different lighting conditions and try to judge if it prefers more or less light (watch for color changes, polyp behavior, growth rate, etc.). The unfortunate truth about coral lighting needs in captive systems is that it's largely guess work (with a lot of trial and error). Always keep in mind that corals in an aquarium are usually already stressed, subject to environmental conditions that are less than ideal and often rapidly changing. I've noticed that the people with the most adaptable corals (the ones that seem to thrive no matter where they are placed), are the ones with systems with the highest water quality, meticulously maintained. Long story short, the most diligent and dedicated aquarists can get away with a lot more than the rest of us can. Thus, if you're not sure what to do about lighting for your coral, try your best to do everything else as well as you can and hope the coral will adapt (it might likely, all else being ideal). [Note: If a coral starts to "burn," move it to lower light. If it bleaches, increase feeding until the color returns.] Part 2: How much light does a particular lighting unit produce? This is another very difficult question to answer accurately. When it comes to lighting units for marine aquariums, we're usually stuck with generalities. Generally, Metal Halide lighting units produce more light than VHO lighting units. Generally, a 250 watt unit will produce more light than a 100 watt unit. However, if you want to get precise and "technical" there are enough other details/factors here to consider to make your head spin! For example, the type of reflector you use can have a great impact on how much light is (or isn't) redirected into your tank. (See Articles: Analyzing Reflectors Part I by Sanjay Joshi and Timothy Marks) It's also prudent to note that corals can only make use of a certain spectral range of light (light with wavelengths between 400 to 700nm). This type of light is measured in PAR (i.e. Photosynthetically Active Radiation). Most of the light produced by our aquarium fixtures is within this range. However, that still leaves the question, how much of photosynthetically "usable" light (how much PAR), actually penetrates the water far enough to reach our corals? Factors such as water clarity and turbulence at the surface make a big difference. A good place to start is to gain a true understanding or what light is exactly (and how it's measured): Facts of
Light, Part 1: What is Light? by Sanjay Joshi When wanting to know how much light your lighting unit is producing, you could "answer" this question very simply (and largely meaninglessly) with a lux/PAR meter (a device used to measure apparent light intensity-- arguably, a rough estimate of PAR) held directly under the light source. However, again, you can assume that the light coming directly out of your fixture is nothing like the light that actually reaches the coral. Light changes drastically as it travels through water (both in intensity and nature). Good luck trying to find an affordable lux meter that can be used underwater! [Note: If you do have the cash for it, the Apogee MQ-200 PAR Meter funs for about $350.] Returning to generalities, generally, light that reaches your coral will be less intense and of shorter wavelengths (i.e. "bluer" or "cooler" light). This has led some aquarists to assume that it might be better (more effective) to start with a light that is "bluer" (i.e. that produces light of shorter wavelengths). Thus, they buy bulbs with higher K ratings. To gain an understanding of why this isn't necessarily a good idea: Facts of Light, Part 4: Color Temperature by Sanjay Joshi Shorter wavelengths of light are of a higher energy (and thus require more power to be produced). Therefore, a "bluer" bulb (higher K rating) might end up producing a lot less PAR because all that energy is being used to produce those shorter wavelengths of light. For what it's worth, natural sunlight is typically <6000K. This article might leave you feeling as if you know less now than before you read it. Well, as the saying goes "the more you know, the more you know you don't know." But I'll leave you with a long list of excellent articles by Sanjay Joshi, exploring these lighting topics in depth (and detailing his research and investigations into light produced by different bulbs). Happy Reefing. Reef Lighting Page by
Sanjay Joshi
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