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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.


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