# Re: Lighting question

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>I've seen references to the effect that lighting requirements are
different if
>the tank is in excess of 22 inches in depth.  I need some clarification
on
>this.  Is the 22 inches from the water surface to the substrate or from
the
>light itself to the substrate or what???  If there is a different
requirement
>for depth over 22 inches, what is that requirement?

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There are a lot of things that come into play that greatly effect the
amount of light that will be put into your tank with any light fixture.
The way to calculate this is quite complicated and the information you
need is generally unavailable. You need to know how much light your
lamps produce, the ballast factor of the ballast you are using with your
lamps and how efficent your fixture configuration is at getting light
produced by your fixture from above the water surface to below the water
surface.

When a tank is deep the situation becomes even more complicated. The 2'
rule probably has to do with the fact that with NO fluorescent fixtures
cannot produce high light levels at the bottom of deep tanks. This is
because the lamps must to spaced very close together to have a chance at
producing enough light and yet when you space them close together you
reduce the fixture efficiency. In a 2' deep tank NO lamps have to be
about 2 1/2" center to center to produce high light levels. This is past
the point of diminishing returns for lamp spacing. For instance, if you
have 9 lamps over your tank in a 2' wide space adding one more won't
give you any more light. Adding 2 more will probably give you less
light.

The solution is to use MH or overdriven fluorescent lamps like VHOs, PCs
or overdriven NOs.

Deep tanks also have the problem of shading. Unless you have only very
short plants at the bottom of the tank the taller plants will shade the
shorter plants. The deeper the tank the worse the problem. You ened up
with way more light at the top levels of the tank than at the bottom
level of the tank. Fixtures that are able to direct light straight down
will work the best to solve this problem. This means MH and AH Supply
PCs.

Wayne

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