Reef Tank Lighting
First some definitions and acronyms.
General Category Ballast – the device that provides the proper starting voltage for ignition, current limiting, and normal operation to light fluorescent and HID bulbs.
Magnetic ballast – Older style, wire wound metal core. Used extensively in the USA.
Electronic ballast – Newer style as the name implies, more efficient and produces less heat than magnetic ballast.
Kelvin temperature or rating – The Kelvin color temperature of the light source is the color of light emitted by a "black body" when heated to a certain temperature measured in degrees Kelvin. Basically what it means for an aquarist is that the higher Kelvin Rating of 20,000 is blue in color and 6500 degrees Kelvin is about as low as you want to go for your reef aquarium. It is a yellow white color in appearance.
Lumen – A unit of light measurement. What you will see on the packaging for almost all bulbs to measure the light output of that bulb. A loose definition would be the total amount of visible light emitted.
PAR – Photosynthetically active radiation - a unit of measurement for light from 400 to 700 nanometers that is useful to plants in the process of photosynthesis
Actinic Bulbs – These bulbs emit light in the 420nm to 460 nm wavelength and cause corals to glow. Sometimes called the blue bulbs and the purple bulbs.
Fluorescent Fixtures.
PC or CF stands for Compact fluorescents or power compacts. These terms are generally synonymous and stand for the newer fluorescent small tube bulbs and associated fixtures.
They are greatly improved over the older fluorescent tubes that we see in the home.
T12 T8 T6 T5 The diameter of the fluorescent bulb in bulb in eighths of an inch. So, a T12 bulb would be 12/8 of an inch in diameter a T5, 5/8” and so on.
NO, HO, VHO – Normal Output, High Output and Very High Output. These are fancy terms the light bulb manufacturers came up with to describe the fluorescent bulbs.
HID Fixtures MH followed by U or H stand for Metal halide which is also referred to as a HID -
High intensity Discharge the U stands for Universal mounting position and the H stands for a Horizontally mounted bulb. The H bulbs should not be mounted vertically in a pendant.
HQI – Hydrargyrum quartz iodide. A different type of metal halide that is usually double ended and requires an ultra violet shield and a higher output ballast to operate.
SE – Single Ended – a bulb with a single connector on the end.
DE – Double ended – a bulb with a connector on each end.
Here are some of the different Types of light fixtures and how they might apply to a reef aquarium. These are generalizations that will work most of the time. This does not mean that someone hasn’t kept SPS corals under 110 watts of NO lighting for 5 years with great growth.
Normal Output Lighting – Fluorescent – Used mainly for fish only aquariums. They are perceived not to have enough usable light to keep corals.
Compact Fluorescent or PC lighting – Used mainly in tanks with a height of less than 14” Can be used in short tanks for corals the require moderate to low lighting.
VHO Fluorescent lighting – The older standard of reef tank lighting before the availability of good metal halide lighting. Can be used in sufficient quantity for clams and some sps corals. Works for most moderate to low light corals in tanks 20” or shallower.
T5 HO lighting – 5/8” diameter newer style fluorescent lighting. What some would consider to be as bright as and better than some HID lighting. In sufficient quantity they can be used for all light demanding corals and clams.
HID or Metal Halide Lighting – Considered to be the standard for all reef tanks of just about any shape or size. General rule of thumbs
1. One metal Halide bulb for each two feet of width and length of aquarium.
2. 175 watt bulbs or less for tanks in the 12” to 18” range depending upon the type of corals that you are keeping
3. 250 Watt bulbs for tanks up to 22” deep
4. 400 watt bulbs for tanks deeper than 23”
5. Color temperature or Degrees Kelvin of the Bulb – 20k bulbs are blue in color and generally have a lower PAR value than bulbs with a lower Kelvin rating. They generally do not require actinic supplementation for the corals to glow. 10K bulbs are white in color and generally have a higher PAR value than the higher K bulbs. Actinic supplementation is desired with this type of bulb.
6. A 10K HID Bulb from one manufacturer does not always look like a 10K bulb from another.
There are four types of ballasts to ignite and supply power to Aquarium HID bulbs
1.
Probe start ballast – Starts and runs HID bulbs with the ignitor in the bulb
2.
Pulse start ballast – Starts and runs HID lights with an ignitor in the ballast
3.
Electronic ballast – Starts and runs all types of HID bulbs
4.
HQI ballast – Starts and runs HQI HID bulbs – It can also be used on the other two types of bulbs by overdriving them. This could
possibly lead to shorter bulb life.
Reef tank Lighting 
