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Re: Color spectrum question

Chester Wong wrote:

My current vision is to have an open top tank full of aquatic plants.  I
want to incorporate my other hobby--orchids.  I would want to place orchids
either at the base of the tank or along the top perimeter, depending on
requirements.  I have a bunch of orchids now and they really thrive outdoors
NY during the Summer, but they are starting to fade since I've brought them
indoors.  So I would like to buy a light fixture soon to give them some

I originally wanted to go with Metal Halide as I love the color and wave
effect.  I proposed my idea to the orchid folks on the newsgroups and have
told that a combination of Metal Halide and High Pressure Sodium would be
for the growth and flowering of plants.  Apparently the increased light
in the red spectrum is essential.

My question is if this is counterproductive to aquatic plants.  Will I
growth of algae or some other negative effect associated with HPS?

I reply:

This spectrum should work fine with aquatic plants although placing your
fixture so far away may cause problems geting enough light into your tank.
HPS is a very very efficient light source. Even though the light produced is
not centered on the part of the spectrum most sensitive to the human eye,
HPS still produces the most lumens per watt of any other lamp. This means
those lamps must produce much more plant usable light than either MH or
fluorescent lamps. Therefore placing the fixture so far away should be at
least partly offset by the superior efficiency of the HPS lamp.

I think the main reason that they are not normally used in aquariums is that
HPS alone produces a very yellow light that most people find quite
unattractive and they have very poor color rendering.  The MH lamps will
help to solve this problem but not altogether. I would be a little concerned
that the light on one side of the fixture will be significantly different
than the light on the other side of the fixture which might produce a poor
visual effect.

I like your plan. I would definately get the unit that incorporates the HPS
and the MH lamp in the same fixture that way the lamps are closer together
and the light will mix better. I don't know of a single person that has
tried this setup but I would be willing to bet your money that it will work
very well :-)

The main problem will be trying to determine how much wattage to buy. This
depends on the beam spread of the fixture and the light output of the lamps.
For the type of lamps you have I think that you can safely use lumens to
calculate the light intensity because the lamp combination  in question is
basically full spectrum. If anything, using lumens will under rate the
lamps. So, for planted tanks I think that something in the range of 200
lumens per gallon of full spectrum light hitting the surface of the tank
will definately be enough to grow most plants. The light hitting the surface
of the tank should be somewhat proportional to the area of the tank divided
by the area that the fixture will illuminate when you set it at 3' above a
flat surface times the the fixture efficiency times the lumen output of the
lamps. You can probably assume the fixture efficiency is about 80%. So what
you really need to know is how big a spot that fixture will produce at 3'
above the surface. If it is 6' by 8' then  you won't have enough light in
the tank for many plants. If the area of illumination is 3' by 5' then you
will have lots of light. It's hard to say without more information.
Basically I guess that if you plan to illuminate a 3' by 5' area with the
500 watt fixture then your tank will do well almost regardless of it's size
unless it is very deep, if you plan to illuminate a much larger area then it
is less likely to do well.

Will orchids be able to withstand that much light at the surface of the
tank? Do they like shade? If they can't you can still have a low/medium
light tank at lower light levels or you can use a very shallow tank.

Thanks for bringing this fixture to my attention. It looks like a good deal
to me.

Wayne Jones

My DIY planted aquarium homepage: