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Re: Metal Halide - can you have too much light ?

Date: Mon, 5 Feb 2001 23:42:13 +0800
From: Jon Hammond

> My question to you is can you have too much light ?  My
> tank is 200 Litres and is a 3ft tank and the metal halides
> I can get are 175 watt or 250 watt for exactly the same
> price ? Will the 250 watt be overkill and are there ill
> effects from having heaps of light ?  What should I do
> get the 250w or 175w

You can definitely have too much intensity. Too much light can cause your
plants to believe they're at the surface, producing emergent growth patterns
while still 6-10 inches under water.

Why do I say this? When setting up one of my 55-gal Show tanks, I decided to
use 100 W Mercury Vapor lamps. While building the "hood" for them, I simply
sat the lamps, which were in reality $30 yard security lights, directly on
the tank top, about an inch- and- a- half from the water's surface. But I
left it there for a while when I started noticing certain changes in the
plant growth and wanted to observe further.

The tank itself was the one I placed in the Showcase section of the AGA's
IAS&C. The effects of the intensity can be seen in the set submitted. As I
wasn't planning on competing with the tank, I used it to demonstrate ways of
coping with "on-hand" situations while aquascaping (an in-depth article on
the tank is in the works). In the meantime, I still have a layout set up at
for visual reference to this text.

The Hygrophila difformis in the left rear corner refused to grow upright.
Instead, it began to simply carpet the tank bottom. This effectively crowded
growth up against the Ludwigia species, causing them to grow forward in an
arching curve, which helped to create some depth to the "grotto". I liked
that, so I kept it.

The Rotala macranda in the left front didn't used to be there. Limnophila
sessiliflora did. The intent was to blend one upright, larger leafed plant
into another more finely leafed one as you approached the front glass. But
the Ambulia would shift into emergent growth and flower while still six
inches under water. Made it look ragged as all get- out, so I replaced it
with a more light- loving plant.

The extra intensity also helped to keep the Cryptocoryne wendtii "brown" in
a ground- hugging mode rather than an upright forest. That was OK too,
considering its position.

The middle of the tank is covered with 24" Gro-Lux, so you have to move to
the right end f the tank for the othe MV.

Notice how _short_ the Cabomba caroliniana is? And still _flowering_?

Much above its present level and it would turn horizontal, as if trailing
the water's surface. And instead of the usual side shoots, it produced a
plethora of pad stalks. Hmm...not quite the effect I had intended. But I
discovered that if I kept it fairly short, I could get both fuller
_submersed_ growth behaviors *and* a pad stalk or two. So I timed one out to
bloom for the picture set.

On this side, you can see some more of the C.w. "brown" that _is_ growing
upright. It's a bit shaded by the Hygrophila polysperma "Rosanevig" (Sunset)
behind it.

The Echinodorus "Red Flame" to its immediate left, however, is under the
"blazing sun". Rather than "waste" any energy on large, lush growth, it
concentrates on sending out a constant stream of flower stalks.

That whole tank is completely refitted with 48" Gro-Lux now. As soon as the
growth patterns have finished shifting again and things are more
"normalized", a new set of photos will be taken for comparison.

As to the question of whether a 250 W Halide is too powerful for _your_
tank - well, to start, I'd guess that you aren't *about* to set the lamp
directly on the cover glass. So enough said on that.

But you will have to raise the lamp a good bit in order to get coverage
across a three- foot length, in which case you approach an old hydroponics
rule of thumb we used to use: "A foot on a side per 100 Watts". This
translates to 1000 W lamps covering a ten- foot square area, a 400 W to a
four- foot square, etc.

Setting a 250 W lamp high enough to get sufficient coverage should be just
about right for your tank. And you may have to live with a good deal of
"spillage", depending on hood/reflector design. (I tend to use spillage to
grow potted plants around the tank.)

Or you could concentrate the lighting from the 175 W to cover most of one
end of the tank, supplementing the other with a small fluoro or two for a
muted, "shaded" appearance and a tighter rein on spillage.

> Another concern of mine is its from a hydroponics shop
> and the tubes they come with are rated at 6200K.. is this
> ok for planted aquarium or do I want to try and see if I
> can find something closer to 5000 K ?

If it were me, I'd go with the 6200 K.

5000 K lamps are meant to come closer to "noonday" whiteness, but some
people still show a preference for "Ultra Daylight" type lamps in the
mid-6s. They report the 5s as still a little too "green" for their tastes.
So manufacturers blend in a little more blue and red to "offset" the green,
and this is good for the plants. The key is the "blend" of spectral spikes
they use to achieve the appearance of "whiteness". They aren't continual
spectrum- types of lamps, but cluster their output at specific wavelengths,
and if you vary the wavelength and intensity the combination shows  varying
degrees of "white".

The advantage of buying from a hydroponics store is that they normally have
the spectral charts available for each of their lamps. If you _can_ compare
the two, look for the one that puts out the highest combination of 420-440
nm and 610-660 nm - the blue and red portions. After that, it's more a
matter of your own personal preference as to output color "cast"...


David A. Youngker
nestor10 at mindspring_com