Metal Halide Light Guide
How metal halide HID lamps deliver intense, penetrating light with a shimmer effect for deep reef tanks, their high power and heat, and why LEDs replaced them.
What it is
A metal halide light is a high-intensity discharge (HID) lamp. In aquariums it produces intense, deeply penetrating light and a characteristic shimmer effect on the water surface that is prized in reef aquaria. It was a mainstay of reef lighting before LEDs became dominant.
Spectrum
Metal halide lamps are available across a wide range of color temperatures, from about 5000 K to 20,000 K, allowing aquarists to choose a warmer, more yellow appearance or a much bluer one. Higher Kelvin lamps look bluer, which suits reef displays, while lower Kelvin lamps appear warmer. The light supports photosynthesis by the zooxanthellae that live in reef corals, the symbiotic algae that supply much of a coral's nutrition.
Light quality
As a single intense point source, a metal halide lamp casts a focused beam that penetrates deeply into the water and creates a shimmer effect on the surface and substrate. This shimmer and strong penetration are the main reasons some reef keepers still prefer metal halide over more diffuse sources. The same point-source nature, however, means it does not give the balanced, even coverage of multi-tube or multi-LED fixtures.
Power and heat
- Metal halide lamps use a lot of electricity, with 150–400 watts being common.
- They produce copious amounts of heat.
- Fixtures are often mounted at least about one foot (around 30 cm) above the tank.
- Some setups require a chiller to offset the added heat.
Comparison with LED
LED fixtures are generally the most efficient aquarium lighting and produce less heat, which is why metal halide has been largely replaced by LED. However, some aquarists still favour metal halide for its natural light quality and penetration.
Use cases
Metal halide remains in use mainly over deeper reef tanks where strong penetration to the bottom is needed, since light in roughly the 400 to 500 nm range drives photosynthesis in the corals' zooxanthellae. Its intense point-source light is valued for the shimmer it creates, and purists continue to use it for what they regard as its natural light quality even though LED fixtures are now the more common choice.
Sizing
Metal halide is typically applied to large saltwater tanks, broadly in the range of roughly 200 to 1000 litres, where its deep penetration suits taller water columns.