Aquarium Lighting Glossary

Aquarium lighting has many purposes, from basic illumination, to color enhancing, to providing energy for photosynthesis. Here are the definitions to some commonly used terms, to help you better understand aquarium lighting.

There are a few terms that you will commonly come across when you are looking at Aquarium Lighting Systems, and Aquarium Bulbs & Lamps. This is a brief glossary of these common terms, to help you better understand the terminology, and help you purchase the proper products for your aquarium.


The power rating for aquarium lamps and bulbs, shown in Watts, can be looked at as a measure of both the power consumption and output of a light bulb. The higher the wattage, the higher the energy consumption, and with respect to each type of light bulb, generally the higher the wattage, the higher the output of the light bulb. Newer technologies, such as LED Lighting, produce much more light output per Watt, versus older technologies. Light fixtures are designed to use a specific type and wattage of bulb, and should always be replaced with the same.

Kelvin Rating:

Without getting too technical, Kelvin rating is a numeric scale that describes the color of an object at a given temperature. For aquarium purposes Kelvin rating describes the color, or spectrum, of sunlight in a given environment. For example, the approximate Kelvin Rating of tropical sunlight at sea level is 5500K. Sunlight is comprised of many wavelengths of light, which combine to produce the “white” light that we see on the surface. As light enters water, the different wavelengths of light penetrate differently, and are filtered out as depth increases, and the Kelvin color rating changes. The deeper you go into the water column the higher the Kelvin Rating becomes. 20,000K light bulbs are designed to mimic deep water environments, and are very blue in appearance. Daylight spectrum (5000K-6700k) bulbs are ideal for freshwater aquariums with live plants, higher Kelvin bulbs (6700K-20000K) are ideal for saltwater aquariums.


For Aquarium purposes, the term Actinic is used to describe the blue spectrum of light, from about 420nm - 460nm, which is beneficial to photosynthetic corals and invertebrates. Actinic light will also enhance fluorescence in the tissues of corals, fish, and other objects with fluorescent pigments. Actinic lights are commonly used to enhance colors, as well as simulate dawn and dusk lighting schemes in marine aquariums, but can also be used in freshwater aquariums to enhance the colors of certain fish. Blue light penetrates the deepest into the water column, actinic lights are used to help simulate deep water environments.


T-rating refers to the diameter of Fluorescent light bulbs, the scale is based upon 1/8th of an inch. For example, a bulb with a T8 rating has a diameter of 1 inch. The two most common aquarium bulbs are the T5 and T8 bulbs (T12 bulbs have been discontinued). T5 and T8 fluorescent bulbs have different pin spacing and wattage, they are not interchangeable.


The Ballast is the electrical component of a fluorescent or metal halide lighting system, which converts the incoming electrical supply, into a form that enables the light bulbs or lamps to produce light. Ballasts are specific, for specific types of bulbs, and must be matched accordingly. Ballasts can be internal or external to a light fixture, and can be included in a lighting system, or sold separately as part of a kit or replacement part.