Aquarium Light: Why, How Much, & How Long

Aquarium Light: Why, How Much, & How Long

The aquarists balancing act between having enough light and too much.

In the beginning of this article we discuss in general terms why light is required in the home reef aquarium. In later sections we will dive a bit deeper into the importance of light and its impact on Zooxanthellae Algae. To get the discussion going I’ve posed two questions/answers below:

1) Do fish need light?

Answer: Simply stated, no! It’s likely fish receive nutrients from the suns rays the same way land dwelling animals do, however, there is a family of fish called Amblyopsidae (Cavefish) who are able to thrive in areas with minimal, or even without, visible light.

2) Why do we care about light in the reef aquarium?

Answer: Coral requires light to perform photosynthesis. Through photosynthesis corals are able to produce the nutrients and energy needed to support growth. Too much light, or not enough, can cause coral to discolor, stop growing, or even die.

The main factors to consider when lighting your aquarium are:

A. Photoperiod – Duration of illumination
B. Intensity – Amount of light energy produced (Typically in PAR value)
C. Spectrum – Wavelengths of light used in aquariums (400nm – 780nm)

To properly understand the importance of light we must first consider why artificial light is needed. Light is an integral part of the reef aquarium because corals have large populations of Zooxanthellae Algae throughout the corals soft tissue. Data gathered by Red Sea Corp. tells us that Zooxanthellae Algae produce as much as 85% of the energy used by coral while the remaining 15% comes from directly metabolizing nutrients from the water.

Zooxanthellae use photosynthesis to convert sunlight into carbohydrates, amino and fatty acids. The energy produced gives the coral the necessary fuel to run metabolic processes like protein production and skeletogenisis. In turn the coral produces nutrients, nitrogenous compounds, phosphate, and CO2 that the Zooxanthellae use to grow. This symbiotic relationship proves beneficial to both parties.

One important thing to remember is that even though there is a cohesive relationship between Zooxanthellae algae and its host coral there is still competition between one another. In high nutrient & waste systems the Zooxanthellae population will grow quickly. The flourishing Zooxanthellae can grow large covering the remaining portion of the coral. This typically leads to the coral becoming darker with deep brown tints obscuring the natural color of the coral. Novice aquariests will often attribute this discoloration to improper lighting when the issue is truly based on improper nutrient levels.

Now that we understand why light is required for coral to grow now lets discuss how much light is needed. Unfortunately there is no single answer. All corals, including corals within the same family, have different photoperiod, intensity, and spectrum requirements. Additionally third party variables such as the type of lighting system, depth of tank, clarity of water, coral competition, dosing and feeding schedules all play a factor when choosing the amount of light needed for the home reef aquarium.

Lets consider how much sunlight corals receive in the wild. The Reef Aquarium Science, Art, and Technology (Vol. 3) tells us that the maximum value of sunlight output in Hawaii is 2200 umol/m^2/s. In turn the reef itself receives around 50mol/m^2 of light energy. If the hobbyist were to recreate an equivalent amount of light output then the home aquarium would receive around 100mol/m^2 of light energy. Meaning, that by replicating the output of the sun in an aquarium that enclosed reef system would receive twice the amount of light energy that would be found in nature, but why? The reason less light gets to the coral in nature is diffusion. Clouds, sediment, tides, seasonal changes in sun elevation, and competing species all play a roll in diffusing light in nature.

To help standardize all the available lighting options Mid-Cities Aquariums has developed are general lighting guide below. It’s important to note that these suggestions are based upon our years reef keeping experience. The standards provided below are based on a commonly available 90G (48” x 18”) Reef Ready Aquarium.

A. Photoperiod

Hours of Light

Light Color












Note: Studies show lunar cycles may play an important role in livestock behavior

B. Intensity


Light Fixture




150W (x2)

250W (x2)







C. Spectrum


Light Color













Leave a Reply

This question is for testing whether or not you are a human visitor and to prevent automated spam submissions.