Extracts of an article by Karen Randall
Some aquarists make lighting for the planted tank into a very
complicated issue. The fact of the matter is that plants are very
adept at using whatever light is available to them as long as the
intensity is sufficient and the photoperiod is adequate.
While it is true that plants use certain parts of the spectrum
more easily than others, they are capable of using light from an
amazingly large portion of the spectrum. Light from the blue end of
the spectrum encourages short bushy growth, while light from the red
end encourages long lanky growth. Plants are least able to use light
from the green portion of the spectrum.
Because most of our plants come from tropical or subtropical
areas of the world, a photoperiod of 10 to 14 hours a day is
appropriate. I run most tanks on a 12-hour photoperiod, but if you
are having algae problems, you might want to limit the photoperiod
to 10 hours, as long as the intensity is adequate. Beyond a 14-hour
photoperiod, most higher plants stop photosynthesizing anyway, even
in the presence of light. In fact, there are many plants (Rotala
wallachii is a good example) that actually close up their leaves
when they have had sufficient light. Algae, on the other hand, is
very capable of benefiting from an extended light period, and can
cause all kinds of problems in a live plant aquarium.
Of course, we can easily control the photoperiod we provide for
our plants. A little research will provide you with information on
the spectral curve of any particular lamp you use for your tank.
What is harder to quantify is the intensity of any particular bulb
or tube. And, intensity is the third, and most critical, part of the
lighting equation.
It's just about impossible to over-light an aquarium with
standard fluorescent tubes. While there are definitely some tubes
that put out more light than others, when speaking about standard,
high output (HO), very high output (VHO), compact fluorescents (CF)
or metal halide (MH) lights, the amount of light they put out for
the amount of electricity used is in the same ballpark. For all
these types of lighting, 2 to 4 watts per gallon will produce
adequate growth. This rule is not applicable to incandescent or
other types of lighting that use more electricity to produce smaller
amounts of light, but these other types of lighting have limited
application in the aquarium anyway for a multitude of reasons.
If you are trying to squeak by with the smallest number of tubes
possible over a tank, it's important to look for tubes that will
give absolutely the most plant-useable light for the electricity
consumed. For most of us, I think it is less complicated and more
cost-effective to shoot for something in the mid range as far as
watts per gallon is concerned, and not worry about paying big bucks
for the very fanciest tubes available.
Another point that needs to be factored in when appraising a
particular bulb is re-lamping costs. We do not operate our
fluorescent tubes under optimum conditions. Most aquarists still use
tar ballasts, and we run the tubes under conditions that are much
warmer and more humid than they were designed for. Because of this,
the tubes degrade more quickly than they do under manufacturer test
conditions. The problem with using fluorescent tubes until they
completely wear out is that they have lost much of their light
output long before then.
Most aquatic gardeners agree that it is best to replace
fluorescent tubes on a rotating basis so that no tube over a tank is
more than one year old. For instance, if you run three tubes over a
tank, change one every four months. This will give the most
consistent illumination.
Whether you use standard fluorescents or one of the higher
intensity forms of light is a matter of the dimensions of your tank
and your own personal preference. A "normal" (not a deep
or "show") tank can be well lit with standard
fluorescents. With tanks deeper than about 20 inches, or those where
the footprint (length and width) is significantly smaller than the
height of the tank, like hexagonal tanks, it is often necessary to
go to a more intense form of lighting. If the tank is long enough,
HO or VHO lamps may fit the bill. For tanks that are shorter in
length, or those that do not easily accomodate the standard 2-, 3-
or 4-foot fluorescent tube lengths, CF tubes or MH may be the best
answer. Most aquarists who want to work with open-top tanks prefer
to use MH lighting because of its greater intensity and more focused
beam.
Once you have decided on the type of lighting you want to use,
you can begin to narrow down the choices of specific tube types. If
you have chosen something other than standard fluorescent tubes,
your choices will be more limited. For standard fluorescents, the
choices are so broad that things again become confusing.
Most of the terms we use for assessing a light are based on human
perception of light. Lumens, lux and color rendering index (CRI) are
all ways of measuring the qualities of visible light. Unfortunately,
the part of the spectrum perceived as "brightest" by the
human eye is the part of the spectrum least useful for the growth of
healthy plants. Also, our eyes are very adept at adjusting to light
sources available, and sometimes give us the impression that a light
is brighter than absolute measurements tell us it is.
The good news is that whatever fluorescent tubes you choose, if
you use enough of them, your plants will grow. Now that I've pretty
much told you that I can't tell you what the best lamps are, I will
give you my personal criteria for choosing standard flourescent
tubes. I want tri-phosphor tubes that have an "M"-shaped
spectral curve — strong in red and blue light, less in the
yellow-green area. I want tubes that have as high a lumen-per-watt
rating as possible. While this is more an eye appeal criteria than
one of plant need, I prefer tubes with a Kelvin (K) rating of
between 5000 and 6500 K. This is similar to the colour temperature
of the sun directly overhead at noon.
I like to come very close to 3 watts per gallon, and I change one
tube every four months. I also have a number of tanks that need
plant-quality lighting. For these reasons, I cannot afford expensive
tubes. This puts most of the speciality aquarium tubes out of reach.
Cost wise, there are other tubes that can be purchased for much
less, they put out an excellent-quality light, and are available
through your local aquarium store.
As a final word on your algae problems, remember that lighting,
by itself, does not cause algae problems. An imbalance between plant
growth, stocking and maintenance, lighting and nutrient levels is at
the root of most algae problems.
