basserman wrote:
Do you think I should change to a 5,000K - 6,000K rated bulb? Does
the
K even matter for plant growth or is that a rating of lamp color
visible to the human eye. From what I've read, it's more about the
red
and blue spectrum that the human eye can't see anyway.

Maybe, yes, yes, right, correct. In that order. Quick physics lesson:

Wein's law states that the peak emission of a blackbody radiation
source is inversely proportional to the fourth power of the temperature
of the blackbody. That means that you can tell the spectrum of a
blackbody by its temperature - or vice versa. So the "K" you see on
lights is the temperature in degrees Kelvin that a blackbody radiation
source would have to be to produce the same spectrum as the light.

So, does the K matter for plant growth? Yes. If you have a light with
an extremely narrow emmission spectrum of say 500 to 510nm and nothing
else then your plants will die very very fast. Luckily aquarium lights
all have fairly broad spectrums so that isn't really a concern. And
incidentally if you had a 500 to 510nm light it would look extremely
bright to you and be the same as no light at all to the plants. Why?
Because that's green light - the peak emission of our local blackbody,
the sun. Our eyes are adapted to see it better than any other color
because it's the color that is coming from the sun most brightly, while
plants are green specifically so that they won't absorb that color - if
they did it would burn them to a crisp. Which incidentally is exactly
what happens every fall, the trees lose their green color and get burnt
to a crisp, the leaves turn crunchy and brown.

That's why it's about the red and the blue. Our sun is between 5000 and
6000K and has a peak emission of green, but since red and blue are
fairly close to green it emits quite a bit of those too. So if you have
a 5600K bulb it will emit about the same light that the sun emits -
which is obviously good for plants. But if you have something like a
3500K bulb you'll be emiting lots of red light that plants can use
which also works...or if you have an 18000K light you'll be emitting
lots of blue light that plants can use, which I haven't personally
tested but I suspect it would also work.

One thing that might be a concern is UV light. As your K gets higher
you produce more of it and most living things don't like that much.
Someone who's tested an 18000K bulb on plants will be able to tell you
if that is a concern or not.

In the end if it looks bright to you and isn't purely green then it'll
almost certainly work fine for the plants. Your eyes are developed to
see the same spectrum that plants are developed to use for
photosynthesis...because we both use the same blackbody to get our
light from; the sun.

-Daniel