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Re: [AGA Member] Light in deep tanks -- some imprecise thoughts

There are two views on this. One holds that, because light
is spread as the distance from the source increases, the
amount of light at the bottom of a deeper tank is less than
in a shorter tank, all other things being equal.
The other view holds that the glass panels, partly due to
the tank being filled with water, redirect much of the
light that strikes them back into the aquarium and downward
so that the rule for distance from the light source does
not apply in the same way as in open air. You can actually
see evidence of this for yourself by turning on the
aquarium lights over an empty tank in dark room, then
filling the tank. The room will be darker when the tank is
filled. Of course, some of the light is being absorbed by
the water. But even with very clear water, the effect is
noticeable. Some early reports on this view, along with
some numerical data are provided in an APD thread ffrom
1995 including Ronald Wozniak, Wright Huntley, and George


And there are more recent posts from Wright and George and
some other guy:




I'll point out that Wright Huntley, included in the above
discussions worked for most of his life (well, so far
anyway) on light and optics, and when it comes to lighting
aquaria, his are views to be reckoned with.

Focus on focus. Not often a focus of the above discussions
is that the angles of the reflector can effect where the
light is most concentrated. By the time you try to factor
in the reflector angles, the angles on reflection between
bulb and glass panels and reflector and glass panels, and a
light meater that doesn't measure the light that plants
like most (the diff wavelengths of light "reflect" at
slightly diff angles as they pass through air, water, into
and back out of glass, and then water before reaching your
plants . . . by the time you try to factor all that in,
Ithink it's pretty hard to say exactly how much diff height

My limited experience is that plants behave just as if
there is less light at the bottom but not as much less as
one might first suspect. However, I find it harder to grow
the light-loving foreground covers in my 2 foot tall tank
than in my 15 and and 20 gal. tanks even thought the taller
tank has the highest wpg. The only way to know for sure is
to measure the light at top an bottom it and I haven't seen
a lot of data on that.

Another issue with deeper tanks is that the chances are
higher that things will be shading other things below. This
just because with more height there is more (plant) stuff
between the top and bottom. And with a high amount of light
at the top, stem plants will tend to be even denser near
the water surface than with less light and therefore tend
to shade their own stems at the substrate more than in
shorter tanks, other things being equal.

So that doesn't answer your question, but maybe it will
help you in looking for the answer,
S-till H-oping (for better data and better answers).

--- Amit Brucker <amitb@gtek.co.il> wrote:
> Hi All,
> Does any one know or have researched how effective are
> fluorescent lights in deep tanks (Over 60cm) ???
> What would be the percentage of light loss in the deeper
> areas of the tank ?

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Field Trip, workshops and more, Nov 12, 13 & 14; 
Marriott Crystal Gateway, Arlington, VA, USA
Details & Registration at www.aquatic-gardeners.org  & www.gwapa.org

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