Lighting Banter and the Problem with "Rules of Thumb" By Eric Borneman I have a pet peeve - "rules of thumb." In the last issue, I briefly mentioned one of the "rules of thumb" involving amounts of live rock used per gallon of tank water. In this issue, I would like to briefly discuss another rule that is frequently discussed and misused: watts of lighting per gallon. Some readers may have gleaned from threads on other forums over the years that this may not be the most accurate way to assess the lighting needs for reef aquaria. In fact, that is an understatement. Make no mistake, I was once a person who also subscribed to x watts per gallon, but I have long since learned the error of my ways - here's why.
One of the first requirements of "proper" lighting is to assess the needs of the plants -- or animals, as the case may be. I am periodically asked the question "What lighting is best?" to which I am forced to reply, "There is no best. What are you keeping?" The reasons for this often unhappily met response are based on several issues which must be addressed to adequately assess lighting needs. The "best" lighting is one the meets or exceeds the requirements of the species present. It is not possible to simply "buy the best" and have it meet the needs of every tank inhabitant possible.
The second consideration is one of logic, albeit a logic rarely employed by the use of the "watts per gallon" rule. A simple example is all that is required to make the obvious even more patently clear. Under the "watts per gallon" rule, for instance, five watts per gallon has generally been assumed to be adequate for most reef tank inhabitants, with higher light corals, such as Acropora spp., requiring perhaps up to 10 watts per gallon. If one has a 10-gallon tank, then, three 18-inch 15-watt bulbs would put the aquarist darn close to an otherwise whopping five watts per gallon. A single 24-inch 95-watt VHO fluorescent light would nearly make the 10 watts per gallon. A single small 150 watt metal halide would put one at a tremendous 15 watts per gallon! Yet, who would attempt to keep Acropora spp., for instance, under a single lowly 150 watt metal halide? Few, I would imagine.
For this reason, it is again important to evaluate the possibilities of the environment, recognizing that certain limitations on the photosynthetic inhabitants of a five gallon tank will certainly exist - or at least not be easily met. However, these limitations should not be interpreted in such a way that very successful reef aquaria with a diverse array of photosynthetic life are not possible. Indeed, they are. The requirement for success need only involve foresight, understanding, and planning for such an environment. Finally, there is the third matter of light attenuation with depth. As many probably realize, light loses strength as it passes through water. "Wimpy" bulbs simply do not have the strength to penetrate deep tanks, and the attenuation can be significant even over a matter of inches. Thus, a 30 gallon "tall" tank may not be well-lit enough near the bottom with bulbs that are adequate for a 30 "long." One hundred and fifty watts of 18-inch bulbs will not have the depth penetration of a single 150-watt metal halide, simply because the sum of the wattages doesn't equal the intensity of the single point source bulb. Along those lines, variations of other aspects of bulb types may not factor out equally. Finally, plants which may have sufficient light energy to thrive near the top of a tank may well not have enough light in the middle or bottom of the tank, despite the uniform appearance of brightness throughout the aquarium -- and despite the "watts per gallon" or bulb type.
It's not so hard, is it? While certain types of lighting are more suited to certain tank habitats and certain plants and animals, and there are aspects of spectra and quality which are met more capably by some bulb types than others, this is an area far less important than aspects of "what and why" light is required. Careful consideration and investigation of the needs of species present, and the desire for future inhabitants, be investigated to assure that one truly does have "the best." Copyright 2002 by Eric Borneman. Used with permission. All rights reserved. Eric Borneman is the author of Aquarium Corals: Selection, Husbandry, and Natural History, and co-author of A Practical Guide to Corals. He is also the co-founder of the Marine and Reef Aquarium Society of Houston and is active in reef conservation efforts worldwide. Other Resources AquariumHobbyist.com has a full line of species forums where you can discuss fish species with other owners, and hobbyists. |
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Lighting is a critical component of any aquarium housing plants which require light for photosynthesis. The reader may ask, "But I thought the corals and other stuff in my tank were animals?!" Well, they are, but the coral animal couldn't care less about lighting - nor could those anemones, sponges, or other marine animals which may engage in photosynthesis. In fact, it is the symbiotic algae ( of many types) which they hold in their tissues that require the light. Therefore, it is actually plants who require proper illumination. This applies equally to macroalgae, mangroves and sea grasses (true plants), and even the frequently problematic pest algae and cyanobacteria (slime algae).
First, most photosynthesizers are adaptive - they can exist well on various levels of light. Other photosynthesizers are best suited to various, possibly specific, levels of low or high light. Others will only grow or thrive under certain light conditions and not others. Furthermore, some gain more from certain spectra of light, not merely being dependent on the amount or intensity. Generally, it is best to choose photosynthetic animals and plants which are either adaptable to the amount of light present, or to choose lighting based on the animals and plants which are desired. This way, they are able to easily provide themselves with various energy rich components of photosynthesis, which are then used for their growth and reproduction.
The point is that bulbs of any type emit a set amount of light. It matters not whether the tank it illuminates is 10 gallons or 10,000 gallons, three 18-inch 15-watt bulbs still will only put out the light of three 18-inch 15-watt bulbs. Conversely, if a coral requires the amount of light put out by a 400-watt metal halide bulb, that same bulb will be needed whether it be over a five gallon tank or a five hundred gallon tank. However, a 400-watt bulb over a five gallon tank will, unfortunately, likely cause tremendous heat problems on a five gallon tank -- unless, of course, the tank has use of a flow-through cryogenic cooling jacket surrounding it!
Lighting is an important topic, but I often feel that too much discussion is spent on topics which are somewhat invalid - such as preferences of one brand of bulb over another or the various "PAR" levels of different HID bulbs. Other aspects of lighting, bearing such intimidating names such a photobiology, photochemistry and photon flux density are enough to send even avid hobbyists scampering for the hills. Yet, these are topics which are the very reasons that lighting is such a critical component to reef aquaria - and to illustrate everyone's capacity to tackle such "advanced" topics, this article has just covered all three topics.