Metal Halide Lights

Metal halide lights are highly regarded for their ability to produce the ideal intensity of light that is most favored by corals.  Indeed, light-demanding tank inhabitants such as small-polyped stony corals (SPS) and clams absolutely thrive under metal halide lights.  Metal halide lighting also gives the closest impression of the mid-day sun directly over the natural reef.

Metal Halide Pendant

Metal Halide Pendant

Metal Halide Wattage

Metal halide lights provide an incredible amount of light intensity in a very small package.  Metal halide bulbs are generally available in three main wattages ranging from 150w, 250w to 400w.

An important point to remember is that the higher the wattage, the deeper the light will be able penetrate the water.  An important consideration if you have a deep saltwater tank.

Metal Halide Lights For SPS Corals

When choosing metal halide lights for an SPS dominated aquarium, it would be best to use either 250W or 400W metal halide bulbs to satisfy these light demanding corals.  Using two or more 400w metal halide bulbs will allow SPS corals to be placed almost anywhere in the aquarium where there is exposure to light, instead of just being limited to the top of the rockwork.

How Many Metal Halide Bulbs To Use Over An Aquarium

One of the most common questions concerning metal halide lights is how many metal halide bulbs should you use over an aquarium?

As a very general rule of thumb you should one metal halide bulb for every 3 feet of tank length.

Which means that you will require at least 2 metal halide bulbs over a 6 foot tank to ensure an even spread of light.  But if more intensity is required, you might even go for one metal halide bulb for every 2 feet of tank, but that will drive your electricity bill through the roof!

It is also a good idea to use the same wattage and brand of bulb when using multiple metal halide lights.  Unless you want one half of your tank to look darker than the other.

Metal Halide Color Spectrums

Measured in Kelvins (K), the color spectrum of metal halide bulbs are rated between 5,600K, 10,000K, 12,000K, 15,000K and 20,000K.

5,600K bulbs mimics the color of natural sunlight, giving like the ocean reef at mid-day, just below the water’s surface.  20,000K bulbs look very blue and will give the impression of the ocean at greater depths of 20 feet or more.  Corals will also fluouresce more under metal halide lighting in the 20,000K spectrum .

Your choice of color spectrum for your metal halide lighting is an important factor to consider as it affects how your fish and corals will look in your setting.

Metal Halide Reflectors

Metal halide reflectors are extremely important as they will enable you to get the most light from your metal halide bulbs.  A well-designed reflector will efficiently direct light downwards into your aquarium, effectively increasing light output by as much as 100%.


Metal Halide Bulb and Reflector

If you are purchasing a ready-made metal halide lighting fixture, the reflector will be built-in.  If you are customizing your own metal halide fixture, or choosing metal halide pendants instead, then a variety of reflector designs are available.

If you are customizing your metal halide fixture to be placed within the canopy of your aquarium, it is imperative that good ventilation in the form of small cooling fans be used to drive hot air out and cool air in.

Placement Of The Metal Halide Fixture

Metal halide lights runs extremely hot!  If water from the aquarium were to splash the glass pane of the fixture enclosing the metal halide bulb, it may crack or even shatter.

For this reason, metal halide pendants or fixtures are best suspended at least 12 inches away from the surface of the water.  This is to minimize the possibility of water droplets bouncing off the surface of the water from reaching the fixture.

Metal Halide Ballast

The ballast starts up the metal halide bulb and regulates the voltage so the bulb runs at the constant correct rating.  A high-quality ballast will increase MH bulb life by keeping the voltage very constant.  It should also provide safety features like high voltage spike protection and automatically shutdown in the event of bulb failure or excessively high temperatures.

There are 2 types of metal halide ballasts — magnetic and electronic.  My preference is for the electronic ballast because it runs cooler and is smaller and quieter.

Life Span Of Metal Halide Bulbs

Used 10 to 12 hours a day, metal halide bulbs will start to shift in color spectrum and output after about 12 months, at which point it should be replaced.

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