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Often more expensive than any other component of a live plant set-up, the up-front and ongoing investment in a lighting system can be considerable. The actual fixtures, timers, lamps/bulbs may not be the biggest expense; electricity is often more in the long haul.
Light and lighting involves three: 1) Quality 2) Quantity and 3) Duration. Quality concerns wavelength mix and considerations of how similar the light seems and is to "natural" sunlight. Quantity is equivalent to intensity or brightness; a measure of how many photons of usable light energy strike a given surface area in a time interval. The last, duration, is how long the light is on/off. This alternating light/dark cycle is best programmed with inexpensive timers.
There are, as in many aspects of life, two general considerations of what light/lighting can do for you. Functionally light energy sustains photosynthesis and acts on other living mechanisms to make them what they are. Aesthetically, you must determine for yourself what looks good.
There are a handful of ways to 'shine on' your aquatic charges; incandescent, halogen, metal halide, mercury vapor, quartz... currently in the west the best available, most popular forms of lighting available are metal halides and fluorescents.
Metal Halides have a mixed history amongst actual users; with complaints ranging from ultraviolet burns to livestock and aquarists to explosions. If using MH, take care to utilize fixtures intended for aquarium use, proper bulbs, placement, shielding, and adequate ventilation, fire prevention around the fixture.
Despite all their potential pitfalls, metal halides produce great quantities of lumens, and beautiful lighting effects in the water. Used properly by themselves or in concert with fluorescents they are the closely mimic natural lighting for aquarium gardens and other systems. My three gripes with metal halides however stand: 1) Their high purchase and operational costs, 2) Safety for your system, home and self, and 3) The fact that full-spectrum fluorescents will give you the quality and quantity of light you want/need for much less cost and danger.
Be aware that there are metal halides, and other lighting modes that have color temperatures of 5,000 Kelvin or less; you don't want these. They are deficient in necessary light in the blue-end part of the light spectrum. However, this limitation can be made up with adding actinic or equivalent fluorescents.
Placement of MH (and other) fixtures is an important matter. It is desirable to locate the lamps as close as practical to the water's surface to maximize light penetration, reducing diffusion and reflection. But, depending on wattage, your water may overheat, requiring water chilling, fans for the fixture(s)... Here again, it is best to carefully read and heed the manufacturers installation recommendations.
Shielding is a twin threat in the way of thermal transfer and life-damaging ultraviolet rays produced. UV light is arbitrarily classified into three basic types on the basis of wavelength. In nanometers these are:
UV A: 320-400 nm
UV B: 280-320 nm
UV C : 200-280 nm
Some UV "A" range is useful, even necessary for some light-using life; "B" and "C" are dangerous to harmful. Most UV "C" coming from off-planet is absorbed by our planet's celebrated ozone layer (257 nm is the wavelength produced by UV sterilizers). UV "B" can be halted by glass and water, and is very destructive to nucleic acids (life) including photosynthetic activity. UV "A" is effectively filtered out for the most part by special filters and bulb coatings provided by manufacturers Where properly employed.
Another real concern is the potential for explosion from these 'high-pressure' light sources getting splashed, or otherwise ruptured by inconsistent warming. Keep the lamps, fixtures and shields spotless, and clean them only when cool.
Other One more aspect of biology must be mentioned with the use of metal halides; increased photosynthesis. So much intense light can speed up photosynthesis to the point where its effects become unhealthy for the system. High oxygen concentration, nutrient depletion, high pH and more should be avoided. If excess photosynthetic effect is a concern (through monitoring or observation of its results) you might want to reduce or elevate your lighting, utilize CO2 infusion (best with a meter and doser), or thin your plants out. This is the "twin-edge" of metal halide use; yes, it is unparalleled in boosting plant growth; but not without concomitant "costs" of added attention to supplementation and maintenance.
Bulb-Life: for metal-halides is somewhere between 10,000 and 20,000 hours. For loss of spectral integrity and brightness reasons they should be switched out once a year.
Full-Spectrum Fluorescents: Are more appropriate, and the best available technology for practically lighting most aquarium gardens. You will find authors who favor all metal halide or mixed MH and FSF instead; read them, and decide for yourself.
Types of fluorescent lamps and fixtures abound. Surprisingly, the majority of lamps sold specifically for aquarium use are not useful for their intended functions. Terrestrial plant-growing, warm, and "broad" spectrum fluorescents typically produce very little photosynthetically useful light. Be very certain about this; you want full-spectrum lamps. Others will not produce the wavelengths you want/need.
Full-spectrum fluorescents are available in a variety of lengths and wattages in three formats: regular, High-Output (HO), and Very-High-Output (VHO) formats, and compacts. The other-than-regular types require special ballasts, end caps, holders and fixture pins, that burn lamps and phase shift more quickly. Their advantage is greater luminosity; a regular 48" lamp is defined as 40 watts of power, VO is normally 60 watts, and VHO can range from 110-160. Respective luminosities (initially) are 2-3000 for regulars and up to 9,000 for HO and VHO depending on wattage and make.
For all types there are new "electronic' ballasts on the market that you definitely want to employ. These modern marvels utilize solid-state architecture, run much cooler than tar types, extending lamp life, discounting phase-shift, saving electricity, and reducing waste heat production.
Bulb Life for regular full-spectrum fluorescent lamps is @15,000 to 20,000 hours, but due to phase shift and lumen depreciation they should be cycled out every year or so. Old lamps may still come "on" and look like new, but they're not. With electronic ballasts, VO and VHO types can usually go six to nine months. Label all with an indelible marker on insertion and rotate out old for new on a periodic basis.
Intensity: Depending on your tank depth, plant species mix and planting arrangement, you may require more or less light to suit your purposes. There are definitely bright to dim habitats where the plants we use hale from; but it is rare that an aquarium garden will outright die from lack of light. Alternatively too much light has many more pitfalls; enhanced algae growth, induced stringy plant forms, and the troubles of enhanced photosynthesis mentioned above. As you might presume, planning so that more light can be added to your system is a very good idea.
Without getting into light meters and measures of incident luminosity, a general rule of thumb for plants requiring the most intense lighting is 3-5 watts per gallon; less for non-light demanding species like Cryptocorynes.
Waste Heat: Take care that whatever lighting array you settle on that it not make the system heat up appreciably during the light-day. If your hood/top is enclosed, close to the waters surface and not insulated via a glass or plastic cover (which it shouldn't be), you may find the diurnal temperature of the system vacillating dangerously. Fans, elevating the fixtures, and even a water chilling system may be called for.
Reflectors whether they're box, pendant, hanging, side-angled types, must be composed of non-rotting materials, arranged or shielded against the effects of splash and spray, and be... reflective. There are stock and after-market sheets and coatings that will bounce, focus, otherwise redirect the light energy down into the system. If you make your own, employ these to optimize your lighting efforts and heat-shield the rest of the fixture.
Proper light for an aquarium garden takes the form of balancing function and aesthetic against costs to procure and run the lighting system itself. Adequate luminosity, quality and to an extent, duration of light is tantamount to success with photosynthetic life.
The current technology of such systems highlights metal halide and fluorescents of different kinds. These provide desired wavelengths, brightness and good looks at reasonable costs. Both forms of lighting have their adherents and benefits, and many people do best by combining the two.
Stepping up plant growth by enhanced lighting does have its detractions; such boosting demands confluent attention to adequate pH buffering, CO2, and other nutrients; as well as a wary eye for the consequences of high photosynthesis.
Cohen, Sandy. 1997. A new look at lighting: VHO the second generation and things to come. FAMA 9/97.
Frados, Andrew. 1986. The importance of light in the aquarium. TFH 7/86.
Frados, Andrew. 1996. Lights and plants in the aquarium. FAMA 11/96.
Jeffries, Owen. 1995. Metal halide lighting for freshwater aquariums, is it really feasible? FAMA 9/95.
Jeffries, Owen. 1997. (On compact fluorescent, VHO and full-spectrum lamps). FAMA 6/97.
Kutty, Vinny. 1991. Lighting for the aquatic garden. TAG 4(2):3,4/91.
Marsh, Robert. 1996. Making your own aquarium covers. FAMA 5/96.
Osborne, Kevin. 1982. Full spectrum lighting in the planted aquarium; pts. 1,2. FAMA 12/82, 1/83.
Osborne, Kevin. 1986. A top trim for the decorative aquarium. FAMA 10/86.
Osborne, Kevin. 1994. Lighting the planted aquarium. FAMA 2,3/94.
Ostrow, Marshall E. 1979. Lighting your tank. TFH 6/79.
Pon, Richard T. 1996. Light intensity and aquatic plants. TAG 9(4):7,8/96.
Schlais, James F. Build a hood for your tank. FAMA 5/80.
Walstad, Diana. 1991. Window light. TAG 4(4):7,8/91.
1,2) MH lighted plant systems; the first by pendants, the second by a beautiful suspended hood.
3) A picture showing properly remoted MH electronic ballasts. Note the air space around them and their mounting above the floor, out of water harms way.
4,5) Two for-sale fluorescent systems; one a stock Eheim product in Germany, the other a Japanese rounded glass tank and top product.