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advantages and disadvantages of natural light

Well one pretty good advantage of natural light (if you are in a
location where you are fortunate enough to have it available! :-) is
that it is almost free. The cost of lighting fixtures and electricity is
the single highest cost factor for artificially lit plant aquaria; would
you agree? (if we ignore the cost of fish and special fish food. With
sunlight, even live food is CHEAP! Mosquitoes are WONDERFUL. Sorry wrong
list; I'm getting carried away by myself again ;-)

Another advantage is that it's pretty easy to regulate the intensity of
natural light (when you are blessed with lots of it) so that you can
achieve proper lighting levels. You can do the same thing by buying more
fluorescent bulbs or getting a metal halide kit, but adding another bulb
is not always an easy job for the workshop challenged or those of us
without talented hubbies! ;-)

This is a good time to mention a disadvantage. It is easy or tempting to
use too much light with natural light when there is an abundance. High
light tanks _are_ much harder to maintain than moderately or "properly"
lit ones. (Check out Karen's posts in
http://www.cco.caltech.edu/~aquaria/Krib/Plants/Tech/lighting.html#12) I
won't proceed further on that subject since it has already been covered
in detail except to note that our empircal watts/gallon can be
translated into the rough equivalent of lux or foot-candles (lumens per
square foot) using the numbers I quoted in the previous compendium:
foot-candles, lux, lumens, sunlight, PAR
(http://www.cco.caltech.edu/~aquaria/Krib/Plants/Tech/intensorama.html).
I think we need a rule of thumb for converting PAR (or % of noon
sunlight) to watts per gallon or the equivalent guidelines but I'm
chicken to make one up because I know there are lighting experts who can
do this better than I.) 

George mentioned 1000 - 1500 Lux (100-150 ft-candles or 20-30 PAR?)
being the most that aquatic plants might require statically. I suspect
that number is only valid for the lowest light tolerant plants. He also
said that getting that value at the tops of low growing plants is about
equivalent to having 12,000 Lux at the tops, so I am going to suggest
that 12,000 lux is a good target. Assuming that tropical noon sunlight
is around 100,000 lux, that is a 12% shading factor. Dave Hueberts
numbers for the compensation point of high light aquatic plants at 85
PAR is about a 5% shading factor. This implies that we find the optimal
spot for growing plants in aquaria at about twice their compensation
point. I think my advise would be to start with fast growing plants with
a relatively low compensation point (what is H poly, about 25-50 PAR?)
and grow in the tank at 10% shading factor. Alternatively, you could
probably grow a tank of Cabomba pretty fast using 25% shading factor. My
gut feeling is that 25% is going to correspond roughly to our highest
artificially lit tanks. On average, the lighting levels in these tanks
are going to be quite a bit less than the noon/sunny day peaks. During
the hot, cloudless summer of Cebu, you probably need about 10% but
during the rainy fall season, maybe you can go up to 25% or more. That's
where the exposure meter on your camera is going to come in handy to
provide you with a baseline. The rainy season is probably the time to
start an outdoor tank too.

Another disadvantage is that when there is an abundance of strong
sunlight and you are situated outdoors, even with shading devices, you
might get heating effects on your tank. 

[Q] Has anybody kept outdoor tropical aquaria who can comment on this
problem??

The most practical advise I could think of is to employ either active or
passive evaporative cooling. Active cooling is where you use a fan to
blow air on the water, a water chiller or a water flow. Passive is where
you use plants, concrete, shade (open air construction), and neat tricks
like wet cloth evaporation fed by osmosis! The thing is if you build a
small shed to shield your aquarium, the inside temperature of that shed
can get pretty darn hot if it doesn't have a way to get in equilibrium
with ambient temperatures or get below the ambient temperature of the
various gray body radiant sources all around it. Hey, terrestrial plants
do an excellent job of this! They draw water up through their
transpiration system and evaporate it from the leaves as a means to
tranport nutrients. It's also why you suffer from heat in Cebu during
the summer so bad; not enough vegetation.

I have a feeling I'm missing one or two more advantages and
disadvantages but maybe somebody will think of them.

The variability of natural light is often mentioned as a disadvantage
but I contend that this is actually a blessing in disguise. There are
three reasons which are mentioned in the long thread in the krib "Light
frequency / distribution"
(http://www.cco.caltech.edu/~aquaria/Krib/Plants/Tech/lighting.html)

1) the period of low light actually stimulates plant growth!! because:
1a) it permits a temporary dip in oxygen levels or redox potential which
increases nutrient solubility and
1b) plants grow when they are not actively photosynthesizing

2) plants store energy much more efficiently than algae especially
unicellular algae and so algae is at a disadvantage during these dark
periods.

3) algae do not tolerate lowered oxygen levels! Even fish are accustomed
to lower oxygen levels of 2-4 mg/L which is a lot less than what we
subject them too with dissolved oxygen levels approaching saturation.
Maybe we should give our fish anti-oxidants like Vitamin A, eh? ;-)

Uwe Behle and George got into a great discussion on this subject in that
Krib URL I referenced above. Uwe says in the Aug 9, 93 article that it
is suggested by H. J. Krause and Dennerle to have a dark period, either
a noon dark period (Dennerle) or an entire dark day once a week
(Krause). This is something new for all you folks battling algae to
try!!

Steve (another middle of the night ramble: is there some inverse
relationship with light here?)

PS. My SWAG estimate of 5 ft-candles / PAR for sunlight is probably
wrong. I was hoping somebody scientific would correct that number. I'll
bet its closer to 4... It is also the flaw in converting George's low
light requirements into PAR values equilvalent to sunlight.