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Re: [APD] Lighting question for small tanks

Welcome back, Wright. Long time no hear (long time not here?). Anyway, thanks for the clarification. I was thinking of the often invoked rule of thumb that light disperses and the intensity diminishes over a given size of area by the square of the distance between the area and the light source -- and what I was thinking is that the rule is not accurate or helpful for an aquarium under a single point source of light.
Under light bars (so-called "line sources" as opposed to point sources) such as fluorescents, further doubts are raised. Basically, the effect of the depth on lighting levels isn't always what's it's cracked up to be, and with home or office aquaria, where the depth is usuall less than 18" and rarely over two feet, it's just not a major concern for many folks.
However, even with plants unobstructed by other plants, turbidity, or my psuedo-amphibous little nephew, Hopper, new shoots on R. indica, for one example, tend to be redder when growing nearer the top and the lights, suggesting that they are getting more light up there. And that's pretty much where the controversy lies, some saying it doesn't matter and some noticing that it does, but not much.  
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----- Original Message ----
From: Wright Huntley <whuntley at verizon_net>
To: aquatic-plants at actwin_com
Sent: Wednesday, April 19, 2006 11:14:30 AM
Subject: Re: [APD] Lighting question for small tanks

>Message: 1
>Date: Tue, 18 Apr 2006 09:24:53 -0700 (PDT)
>From: "S. Hieber" <shieber at yahoo_com>
>Subject: Re: [APD] Lighting question for small tanks
>To: aquatic plants digest <aquatic-plants at actwin_com>
>Always the controversial one, Nick  ;-)   There is disagreement about how much depth affects the lighting level *in an aquarium*. When filled with water the glass are good reflectors and much of the light that would be dispersed outside an empty aquarium is reflected back into a water-filled aquarium. Put an aquarium in a dark room with no water and turn on the aquarium lights. Then do the same after filling with water (the tank not yourself).
>Yet plants in my tanks behave a bit as if there is more light nearer the top than nearer the bottom.

There usually *is* more light near the top than near the bottom, Scott.

Unless you have high turbidity (hence an ugly tank) the water absorbs 
almost none of the light directly in any tank less than about a meter 
deep. What happens is that plants grow, spread, and interrupt a lot of 
light. They absorb some and reflect some back out of the tank through 
the side walls (so we can see them). That is the normal main loss before 
the light reaches the gravel. Crud on the glass can go a long way toward 
absorbing/scattering light, too. It often is uniform and not easy to 
see. but a good light pipe relies on very smooth walls to give total 
internal reflection. When dirt or algae coat the walls, the internal 
light reflection goes down pretty fast. The substrate itself can absorb 
or reflect light in directions beyond the critical angle, hence out of 
the tank, too.

Let's face it, you want to see your plants and fish, so all the light 
cannot possibly stay confined to the light pipe. This means there will 
be less light down near the bottom due to plant shading, if for no other 
reason. It varies wildly from tank to tank, because some folks are much 
better at growing lush plants than I am.


Wright Huntley - Rt. 001 Box K36, Bishop CA 93514 - whuntley at verizon_net  760 937-2276 (cell) 760 874-2000 (CA) or 941 866-0500 (FL).

Congress shall make no law abridging freedom of speech unless speech annoys politicians. --McCain-Feingold

Now, go read the real First Amendment,

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