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Natural Light?


  1. Natural light intensity
    by krombhol/ (Paul Krombholz) (Sun, 2 Nov 1997)
  2. advantages and disadvantages of natural light
    by Stephen Pushak <teban/> (Wed, 12 Nov 1997)

Natural light intensity

by krombhol/ (Paul Krombholz)
Date: Sun, 2 Nov 1997

>From: Stephen Pushak

>I'll try my question again. Didn't get much response last time.
>How feasible is it to use natural light (no sunlight) to light an
>aquatic plant tank? I have a hunch that its important to get about a
>half an hour to an hour of sunlight.
>Can anyone help me correlate the lux intensity of tropical daylight (no
>direct sunshine) with the typical lux intensity we desire in planted
>aquaria? We'd be looking at a percentage of the arc of blue sky.

>Also, how much does this light intensity vary according to a cloudy day
>and a rainy, overcast day?

I don't have the values in lux, but I remember that sunlight is 8,000 to
9,000 foot candles and that shade on a sunny day with about half the blue
sky contributing light and the other half occupied by a building is around
500 FC.  Perhaps full sun all day is a bit too much, but sun 1/2 to 1/4 of
the day has got to be better than only shade.  I got some of my best growth
ever of Crypts  when I had my 55 gallon and 75 gallon tanks right up next
to south-facing windows that got sunlight most of the day during fall,
winter, and spring.

Paul Krombholz in sunny, bright, Jackson, Mississippi,

advantages and disadvantages of natural light

by Stephen Pushak <teban/>
Date: Wed, 12 Nov 1997
To: aquatic plants digest <Aquatic-Plants/>

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) 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 ( 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".

  1. the period of low light actually stimulates plant growth!! because:
  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.

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