lighting 101

You can have all the water chemistry right but without proper lighting you and your tank inhabitants are doomed. Corals and many invertebrates have symbiotic algae that create food and energy for them. Like plants, they require light for photosynthesis to create the food and energy they and their host need to survive. Understanding lighting starts with understanding the lighting needs of your tank inhabitants and then how best to meet them with the lighting options available. One thing you will realize rather quickly is that there are multiple options and no definitive “if you have this you need that” answers. It is also important to know that keeping lights on longer will not compensate for incorrect spectrum or intensity.

So what is going in this tank?

Knowing what you plan to put in the tank is key to deciding the lighting system you will need. A fish only tank generally only needs light to view the inhabitants. Add live plants or corals and you need to supply them with appropriate light spectrum sources and intensity to allow photosynthesis – in essence you are replacing their natural light source – the sun. And different corals have different lighting needs some liking more intense light, others medium or low. Creating a species light requirement chart for your anticipated tank inhabitants will aid in the decision making process. The chart below is a general guideline but even within these groups there are variations:

Anemones Corallimorphs LPS Corals SPS Corals Soft Corals Zooanthids


Low to


Low to


Low to


High Moderate

to High

Low to


Tank Placement

(general depth)

Sand Bed Lower Middle Upper Lower Lower

All light is not the same – Spectrums

Light is made up of a variety of wavelengths (measured in nanometers, nm)
that translate into different hues (colors) and hence the term spectrum. The
sun produces 3 wavelengths of light:

  • UVA: The visible wavelength of light and essential to photosynthesis. Actinic light is a particular portion of this spectrum (420nm) important to marine organisms. Red, orange and yellow light colors are referred to as warm; green and blues are referred to as cooler colors.
  • UVB: This is the non-visible wavelength of lighting. It is what gives us a suntan.
  • UVC: This wavelength is used for ultraviolet sterilizers which kill harmful
    bacteria. This wavelength is very dangerous to all animals.

The different lengths of light waves do not penetrate through the water equally. Red light is the first to be filtered out and can only penetrate a short distance. Next lost are orange and yellow as light waves penetrate deeper. Greens and then blues will penetrate the water deeper, providing essential energy sources to the algae hosted in coral. Of all the colors of the spectrum blue light penetrates the deepest. And there is a slight difference in penetration between salt and fresh water with saltwater absorbing slightly more energy due to its higher density.

Measuring light and what those numbers mean

When you purchase aquarium lighting you will see different terms that describe the type, intensity, and color of the light they produce. Knowing what these mean is essential to understanding what they will do to and for your tank inhabitants.


The Kelvin Light Temperature of lights is not temperature in the sense of being hot and cold like boiling and freezing water. It comes from the lighting industry as a means to measuring the color temperature of a bulb. Aquarium lighting bulbs are rated by color temperature measured in degrees Kelvin (K) to indicate the hue of a specific light source. Color temperatures over 5500K are termed cool colors (green-blue) and the higher the K value the bluer the light appears. Bulbs under 3000 K are referred to as warm colors (yellow-red) and the lower the K value the more yellow then red light appears. 6000K is where the ratio between red and blue is equal. The human eye mostly sees light around 5500K. A typical summer day is 6500K. Higher value K lights penetrate water more deeply, even more so in saltwater, and the higher the K, the more blue the light becomes.


The nanometer is a unit of length in the metric system, equal to one billionth of a meter. It is used to measure very small lengths of many things, including the wavelengths of light. Remember ROYGBIV – red, orange, yellow, green, blue, indigo, violet? The visible light spectrum ranges from a lower wavelength 400 nm (violet) to the higher wavelength 800 nm (red). Below 400 you find Ultraviolet (UV) wavelength and above 800 is Infrared (IR). Wavelength differences determine how the wave affects its surroundings and is why short-wave UV light can destroy DNA and visible light does not. UVC bulb will spike at about 265 nm, an actinic at about 420 nm, and a daylight bulb about 700N. Different spectra are required for different species depending on their natural latitude and water depths. Most corals contain symbiotic algae (zooxanthellae) that require light in the actinic UVA range (400-550nm ) to facilitate photosynthesis.

LUX – lx:

This is a measure of light intensity over a given area and very important to most corals in reef aquariums. Too low a light intensity and the zooxanthellae do not create sufficient oxygen. Lux in tropical reefs measures between 110,000 and 120,000 lux at the surface of the reef and 20,000-25,000 Lux one meter below the surface. For reef aquariums, look for a minimum intensity of no less than 3,000 lux for light reaching the deepest part and no more than 100,000 to 120,000 as too much light can have adverse effects also. One lux is equal to one lumen per square meter and is measured with a luxmeter. Lux are also expressed in watts/meter2 but there is a different conversion factor for every wavelength, and it is not possible to make a conversion unless one knows the spectral composition of the light.


Stands for PhotosyntheticallyActive Radiation and is spectrum range of sunlight, from 400 to 700 nanometers, needed by plants and zooxanthellae algae for photosynthesis. This range is found from actinic UVA (400-550nm) to near infrared. UVA is the absorption bandwidth of chlorophylls a, c², and the light-harvesting carotenoid peridinin, a pigment related to chlorophyll. There are three main spikes in the PAR spectrum (one 400-550nm, two in the 620-720nm range) All three are generally found in a daylight bulb of approximately 6500K. Most symbiotic zooxanthellae and other green algae need more of the actinic spike in the 465-485nm range. (Most actinic bulbs however are in the lower 420nm except for the newer LED lights which have a more precise 465-485nm blue.) For this reason extra actinic bulbs are often included in reef tanks (for corals and clams with zooxanthellae and limited in freshwater tanks to control blue-green algae growth.


Is Photosynthetically Usable Radiation or commonly referred to Useful Light Energy and is the fraction of PAR that is absorbed by zooxanthellae photo pigments that stimulate photosynthesis. So while certain type bulbs produce PAR, the percentage of useful light bands for your tank inhabitants may not be optimum. And even
though two different lights may look to produce the same light to our eyes, they may be producing a significant amount of light in a range not needed (such as the yellow-green spectrum) or used by your particular tank inhabitants. This is why what you see isn’t what you need, and why even enough of the wrong light can still produce enough of the right light.


Are the measure of how bright or intense the light looks to our eyes; the higher the lumen the brighter the light looks to us. Lumen per watt is useful to know when comparing bulb outputs and is figured by taking the lumens listed by the manufacturer and dividing by its rated watts. This comparison does become deceiving when looking at newer LED lights which have more focused light energy with little light energy lost so a LED light often requires half (of less) the lumens to provide the required essential light energy. Because of this, lumens are only part of the equation when figuring out your lighting needs.


Watts are a measure of energy being used, not energy being output. It is equal to one joule of energy per second. Back when lighting choices were limited to bulbs like T12’s, watts per gallon was a reasonably accurate way to work out lighting needs. Advances in lighting have produced bulbs that spread out more watts in shorter distances. Using the traditional 2-3 watts per gallon for plants and 3-5 for reef as the sole determination of tank lighting needs is really not an accurate measure and is just a starting point, not a rule.


Is Color Rendering Index and indicates how a color will appear under different light sources and is the only agreed upon international system to describe what we really see. If a light source doesn’t change the appearance of light compared to a reference source of the same color temperature, it has a CRI of 100. CRI came about as a way to compare continuous spectrum sources whose CRI’s are above 90; below 90 two sources with the same CRI can render color very differently In selecting proper aquarium lighting it is not an important parameter.

So how do I combine all these to help me make my decision? Over time, the experiences of aquarists and actual testing has produced these observations relating these measures to providing optimal tank inhabitant growth and health:

  • Because coral receives intense, direct tropical sunlight, the focus was once on simulating this with 6500K lights. 6500K lights produce the best growth results for fresh water plant tanks and can be used in reef tanks more successfully when used in combination with other light spectrum sources. Blue actinic 50000K or adjustable/multiple LED can be added to balance and increase depth penetration and first blue spike in PAR if using 6500K lamps in marine tanks.
  • 10000 K bulbs can bee a good choice for better PAR, depth penetration and growth with soft corals, LPS, and SPS (but noted slower growth).
  • 14000 K bulbs penetrate more than 10000K while still providing a useful PAR.
  • 20000 K bulb will bring out the fluorescent pigments in many corals. They are used for this reason and in deeper tanks but are not good as the only Kevin temperature lights in a reef tank.
  • 50000 K is the general Kelvin rating of actinic blue light sources. It is a good compliment to 6500, 10,000, 14,000 Kelvin lights in tanks with inhabitants with zooxanthellae algae.
  • Actinic blue lighting beneficial to photosynthetic invertebrates, is also pleasing to the eye when used to supplement your daylight light choice.
  • Fluorescent and incandescent lights produce a lot of yellow and green light which research shows is mostly wasted energy for SPS coral and freshwater plant needs.
  • Lux or light intensity, is crucial to most reef tanks and a minimum of 3,000 lux at the tanks deepest point is suggested with a 100,000 to 120,000 maximum at the surface.
  • Reef tank inhabitants that depend on the photosynthesis of zooxanthellae algae will best thrive with light that achieves the optimum PAR, which includes daylight from 6500-14000K.
  • Light brightness is not an indicator of the light energy spectrum the bulb produces. For example, LED light may seem less bright but the actual spectral energy output is much higher and can be more accurate to your tank inhabitants needs.
  • So while watts per gallon are a place to start, it doesn’t take other important factors into account to ensure you completely meet the needs of your tank’s inhabitants.
  • It is a common practice among reef hobbyists to combine different types of lighting bulbs to create the proper intensity and spectrum for the inhabitants of a given reef tank. There are many possible combinations so talking with others, researching forums, and experimentation will help you find the right combination for your tank.

Deciphering the different types of bulbs and lights

For a fish only aquarium you can use any type of lighting you wish to. If you are going to be keeping live plants or live corals in the tank you will need a lighting system that will provide plenty of light for the plants and symbiotic algae found in corals to photosynthesize. This can be accomplished using multiple NO output bulbs over the tank or high output lights such as a VHO, PC or T5 fluorescent fixture or a MH system. What does that all mean? Keep reading just a bit further!

Don’t forget to consider expected bulb life and the cost of replacements. Bulbs dim over time and this is not necessarily noticeable to human so it is best to replace bulbs on a schedule to ensure proper lighting needs are met. Also consider how you are mounting the fixtures themselves. Fixtures come as fully assembled, that is ready to go right out of the box, and as retrofit kits. You will find them with mounting legs that let you set the fixture right on the rim of the tank or as hanging kits that let you suspend the light over the tank. Retrofit kits usually work best with a custom canopy, mounting inside with a few screws.


Normal Output Fluorescent (NO): These are the standard wattage bulbs included with many stock aquarium hoods. They use a fluorescent starter to regulate the flow of electrical current through the lamp tube. The linear bulbs are double ended and come in many lengths and watts. Because they produce little in intensity compared to other lighting types they are inappropriate for reef system tanks and are seen more often in fresh water or FOWLR tanks. A starting point would be bout 2-5 watts per gallon of water.

High Output Fluorescent (HO)

These have a higher lumen output than VHO, generate less heat, and are more energy efficient. They require an electronic ballast and come in different diameters, wattage, and lengths. They can last considerably longer, between 2-3 years, with less degradation in lumen output. Option for reef tanks or tanks with live plants. A starting point would be about 1-3 watts per gallon of water.

Very High Output Fluorescent (VHO): Allow you to fit fewer bulbs over
your tank and provide substantially more lumens compared to NO bulbs power usage and intensity is about 3 times that of NO bulbs but they generate less heat than metal halides. VHO bulbs are 1.5″ (T12) diameter linear fluorescents available in different wattages and lengths. They require a 1500ma VHO ballast to operate and you can expect to replace them about every six months. Their higher light intensity and more even spread allows for a greater variety of corals to be kept, including most soft corals, most LPS corals, and some SPS corals. Option for reef tanks or tanks with live plants. A starting
point would be about 1-3 watts per gallon of water.

Power Compact Fluorescent (PC or CFL)

Sometimes called compact fluorescent, they are brighter and more energy efficient than NO fluorescent bulbs. They use a technology similar to VHO but have a ‘U’ shape form and are powered from just one end. Bulbs are single ended and come in 2-pin, 4-pin, and 4-pin straight configurations in multiple watts, shapes, and lengths. They allow for more wattage in a single space. Option for soft and LPS corals or tanks with live plants. A starting point would be about 1-3 watts per gallon of water.

Fluorescent Bulb Sizes: You will see NO, VHO, and HO bulbs labeled as T-2, T-5, T-8, and T-12. The “T” value of a fluorescent light refers only to the diameter of the tube. T-2 bulbs are about 1/4″ (7mm) in diameter, T-5 bulbs are about 5/8″ (13mm) in diameter, T8 are 1″ (25mm), and T12 tubes are largest at 1.5″ (38mm). T12 are the bulbs that look most like what you commonly see office ceiling lights.

Fluorescent Ballast Options: Ballasts used for these type bulbs are either coil and core ballasts or electronic ballasts. Coil and core are what commonly comes with NO and VHO strip lights. Coil and core are heavier, use more electricity, and produce some heat compared to electronic ballasts. Electronic ballasts initially cost more but their lighter weight and electricity savings offset this for many users. Some electronic ballasts are capable of powering NO, VHO, and power compact fluorescent (PC) in various combination.

Fluorescent Tube Color Options: There are many color spectrum available in fluorescent tubes, the most common used being full spectrum or daylight, actinic, and 50/50. Remember that the Kelvin rating used refers to color temperature and not the intensity of the bulb. A 20,000K bulb is more blue than a 10,000K bulb not twice as intense. A combination of bulb colors is commonly used to create the light spectrum needs of the tank.

  • Full Spectrum or Daylight Bulb: Generally a bulb of 6500K used to duplicate typical summer light. 5500K-6700K bulbs are a good spectrum for growing live plants. Some reef aquarists believe that corals look and grow best under light with a more blue coloration. A suggestion ratio is one or two daylight lamps for each 30 gallons of water
  • Actinic Bulb: A bulb that peaks in the 420 nanometer range and emits a fluorescent blue light that promotes the growth of the zooxanthellae algae found in photosynthetic corals and invertebrates; simulates dusk and dawn environment, and is used to accentuate the fluorescent colors of many corals and fish. This lighting is visibly blue and needs a full spectrum bulb to help offset the color distortion. Do this by adding a daylight bulb (around 5,000K-6,000K) to your lighting group but remember too much “daylight” (enhanced red spectrum = lower K value) can promote macro-algae growth. A suggested ratio is one actinic lamp for each 30 gallons of water.
  • 50/50 Bulb: These were developed specifically for marine and reef tanks to create light needed for the invertebrates in reef tanks and to enhance visual appeal. They have a daylight (6700-10,000K) and an actinic (03 blue) bulb in the same lamp.


Metal halides produce more lumens per watt than any other aquarium lighting source making them a popular lighting of choice for many reef aquarists. Metal halide lamps are more directional in nature with the most intense light found in the area located directly below the lamp and the amount decreasing with increased distance from the sides, front and rear. Metal halide lighting replicates the natural reflection effects of sunlight bouncing off surface water adding a “sparkle” effect. They do get extremely hot and because they are preferably placed 12″ above the tank they will require some type of ventilation or a chiller system, to dissipate the heat that will over-warm the tank water. A splash shield is also highly recommended as they can shatter if water gets on the bulb and you do not want glass and mercury in your tank!

They are composed of a metal halide lamp (bulb), ballast, lamp socket and mounting bracket, and a reflector. Reef aquarium metal halide bulbs are generally 10000K-20000K. They are either a single-ended screw type base or double-ended plug style, often called HQI bulbs. Both require specific ballast types for safe and proper operation. A general guide is one bulb for every two feet of tank length using 175 watt bulbs for tanks up to 22″ deep; 250 watt bulbs for tanks 22-28″400 watt bulbs for tanks deeper than 28″. The typical lifespan of a metal halide aquarium bulb is about 12 months. Bulbs are available in a range of wattage and color temperatures.

There are four types of ballasts commonly used: Probe Start, Pulse Start, and HQI Magnetic and are all bulb type specific magnetic ballasts plus electronic ballasts that can be used to drive any bulb type of the appropriate wattage.


This is the newest lighting source to come to use for the reef aquarium. These are low voltage, energy efficient, long lasting, and produce relatively little heat. and consists of small diodes attached to a circuit board. They are available in a wide range of colors allowing for combinations to create almost any desired spectrum; they create exceptional coral coloration. They are used for simulating moonlight or as primary or supplemental daylight spectrum. LED manufacturers are now creating fixtures with upwards of hundreds of LED diodes and adjust the diode color ratios to maximize nm peaks. Moonlight simulation is done with a small number of blue LED’s and placed on a timer; some aquarists are using a programmable timer to simulate a 28 day moon cycle. Options are expanding daily for all tank types.

How long to leave the lights on depends on who’s home

How long you leave the lights on depends on what is living in your tank andwhere; again a reason to research beforehand You want to closely mimic the native environment of the inhabitants. This is where timers can become your tank’s inhabitants best friend; a consistent lighting schedule will make everyone happier and healthier. A fish only tank can have lights on for as long as you like but most people seem to go with 4-8 hours per day. Tanks with live plants should have lights on for about 10-12 hours a day to give plenty of time for them to photosynthesize. Corals and reef tank inhabitants come from tropical regions where there is 8-12 hours of light a day.

The ideal lighting for a reef tank is very subjective in terms of type of lights, spectrum of lights, and how long to have each type on. There are a lot of ideas, theories, and opinions out there. Really, you need to start with the type of corals you want to keep, find out what they need, and build a lighting system from there. That said, here are some general guidelines:

  • A tank of mainly soft corals (mushrooms, leathers, and xenia) will likely do fine without investing in a metal halide system.
  • SPS corals (Monitipora, Hydnophora, Acropora, etc.) along with anemones and clams will need as much light as possible.
  • If you aren’t sure what types of corals you’ll end up keeping, to lean toward higher wattage fixtures so you can cover all needs.

What’s this going to cost to run?

What is your lighting system going to cost you (on your electric bill). Since everyone has different electric rates it is impossible to give an exact figure will cost to run. You can estimate the cost by taking the total wattage of the fixture (add up the wattage of the different bulbs), divide by 1000 to get the kWh of electricity used per hour. Multiply that by the number of hours the light is on in a day (8-12 hours per day in most cases) you will have your estimate per day. Another factor to add in to cost is the type of ballasts you use. Electronic ballasts can help reduce the electrical consumption of a lighting system compared to coil and core.

[ Total Wattage x Total Hours of Light ]     x      Cost per Kw (from your electric bill)       =       Daily Cost


For example: Your electricity costs $0.15 KWH and your system has 500 total watts and is on for 10 hours per day it will cost you about $0.75 per day or $22.50 per month (based on 30 days) to run that system.

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