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>I plan on setting up an open topped 90 gallon tank. I haven't decided on a
>power compact or a metal halide. The question is, Is one, 200 watt, metal
>halide enough for a 90 gallon tank? If not, a recommendation please.
Are you *sure* that 200w light you were looking at is really a metal halide
(MH) light? I have seen lots (and lots!) of books that incorrectly talk
about halogen lights (which are basically improved incandescent lights) as
MH lights. MH lights are *arc lights* that have *no filament*. If the bulb
has a filament than it is *not* an MH light (or any other high intensity
discharge (HID) light either).
Typical MH lights come in these sizes:
Normal mogul-base in 175w, 250w, 400w, 1000w, 1500w
HQI (contacts on the ends of the bulb) in 150w and I think also 250w
HQI is somewhat similar to the large halogens that have the electrical
contacts on the ends, but the HQI will have a little "bubble" in the middle
of the tube and there will be *no filament* in it. The halogen bulbs will
be straight tube in most cases and will have a filament running the length
of the bulb.
I've seen some little 35 and 70 watt MH lights too but they aren't very common.
You do *NOT* want to use halogen lights in most cases. They put out way too
much heat for the amount of light they generate since they are not very
efficient, and their color temperature is typically lower than most would
like on their tanks. For MH lights on a 90 gallon tank I think you'll find
two 175w fixtures to be your best bet. It is common practice to use one MH
fixture per 2 foot square area of tank surface to help keep the light
levels in the tank even.
UNIX Systems Administrator