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Mini Lumenarc reflector 3.jpg
Two Mini Lumenarc metal halide reflectors mounted over a reef tank.


Lighting for reef aquariums can be one of the most confusing aspects within the hobby. Even with the copious amounts of information, deciding on the right lighting for your tank can be quite hard. The two principle types of lighting technologies used in the hobby are metal halide (MH) and fluorescent.



Wattage measured in Watts (W) describes how much power a particular lamp will use. This value is important as it tells you how much electricity the lamp will consume as well as give you a rough indication of how much light a particular lamp will output. The higher the wattage the greater the amount of light.

Correlated Colour Temperature

Measures how closely the colour of a particular gas discharge lamp matches the spectrum emitted by a true blackbody at a particular temperaure. For full details, see: Colour Temperature


PAR stands for Photosynthetic Available Radiation and is light that is available for photosynthesis. Practically, it is defined as light between 400 and 700 nanometres (visible light) and so any light that falls between these wavelengths is PAR. It is an important consideration for marine aquariums that include photosynthetic organisms such as algae and organisms that contain photosynthetic algae such as corals, anemones and clams. Photosynthesis is a source of energy for these organisms and the organisms need to receive enough PAR for long enough each day to meet their energy needs.

For full details see Photosynthetic Available Radiation.

Light on the Reef

In order to understand the importance of light, it is helpful to understand the nature of light on a natural reef.

Light on a natural reef comes from the Sun (obviously). The amount of light reaching the water surface is influenced by the time of year, time of the day and atmospheric conditions such as clouds. The light will be at its brightest when the Sun is at its highest point, which will be around noon in Summer. (Note that on the Equator, the Sun will actually be at its highest in March and September.) When the light is at its brightest, it will have a PAR of around 2,000 to 2,500 μE m-2 s-1, as indicated above.

When the Sun is not at its highest point, PAR at the water surface will be less. Throughout the course of a day, the light gradually builds, peaks around noon, and drops back again. For example, PAR may only be 100 μEm-2s-1 at 7am, 1,000 μEm-2s-1 at 9am but reach 2,000 μEm-2s-1 by 11am. Cloud cover can also reduce the amount of light reaching the water surface. Even short periods of cloud cover can reduce the PAR to less than half what would be received under clear skies.

The above examples are what happens at the water surface. While some of the organisms we keep are sometimes exposed at low tide and so receive full Sun, most are submerged the whole time and some are found at depth. Light is lost as it passes through water. Some is absorbed and some is scattered. The deeper you travel into the reef water, the less light there is. The clarity of the water influences how fast light is lost, but even in clear water, as much as 50% of the light could be lost in the first 5 metres. At a depth of 15 metres, as little as 10% will remain of the light that hit the water surface. This means that organisms at this depth may only receive a maximum of 250 μEm-2s-1 and for most of the day, they will receive less than this amount of light.


Lamp Technologies

There are a number of different types of lighting that can be used for a marine aquarium.

  • Metal Halide - efficient point source of light popular for lighting reef aquariums.
  • Fluorescent - most common and cheapest lighting used in both freshwater and marine aquariums.
  • Light Emitting Diode - technology has now reached the point where they produce sufficient light, currently on the expensive side, but rapidly coming down in price.
  • Skylight - no ongoing costs technique to light a tank.
  • Halogen - inefficient (generates lot of heat) and not suitable for marine aquariums.



Fortunately, most corals have a fairly wide range of tolerance for light and so you will generally do fine with most species - as long as you can provide the appropriate environment in other ways (water flow, etc.).

Shorter lighting periods don't really work all that well for acclimation. If the coral is getting too much light it will just shutdown (or more accurately the zooxanthellae will start to photoinhibit). This means that no matter how short the period is, there will be little or no photosynthesis occurring. You would be much better off increasing the amount of light gradually.

When changing over to more high powered lights, the best method would be to raise them higher. This spreads the same amount of light over a greater area so it isn't as bright. Then gradually lower the lights over a few weeks to give the zooxanthellae time to photoacclimate. If you can't raise the lights, shade cloth or something similar can be used to decrease the amount of light reaching the corals. You can also put the corals lower in the tank.

Note that corals are actually very tolerant of increases in light as long as they are not stressed. Because it is difficult to asses whether a coral is stressed or not, it is safer to acclimate them slowly.

One other issue with brighter lights is it may actually be too bright for some corals. Even corals that come from shallow water reach their full photosynthetic rate at as little of one fifth full strength Sunlight. If and when light gets much higher than this, the zooxanthellae may start to photoinhibit. In the wild, this is not too much of a problem as there is time either side of the peaks of light when photosynthesis can occur. In a tank, however, the lighting is generally constant so if you were to provide that amount of light, there may be no photosynthesis occurring.



What lighting should I buy for my tank?

There is no valid answer to that question as there is no "right amount of light".

Symbiotic organisms like corals and some clams, as well as photosynthetic organisms such as algae, are able to acclimate and acclimatise to a wide range of light levels. This occurs naturally as light levels for a stationary coral will vary from Winter to Summer and on different areas of the same colony. Different colonies of the same species can be found at different depths and so will receive different amounts of light. In some cases one colony of a species could receive 100 times more light than another colony of the same species and both colonies survive and grow.

When planning a tank, you need to consider the corals and other organisms you want to keep and determine the amount of light that will be able to meet all their needs. Within reason, going for more light allows more flexibility than going for less light. That doesn't mean that less light doesn't work, but simply most corals can tolerate higher light and you will get faster growth rates with more light.

A rule of thumb that works well and gives the most flexibility, in my opinion, is to choose the number of lights based on the length of the tank and the wattage based on depth of the tank.

Have one metal halide lamp for every 60cm of tank length. For a tank that is 45cm high, you can use 150W metal halide. For a tank that is 60cm high, go for 250W metal halide. For a tank that is 75cm high, go for 400W metal halide.

The above guidelines will mean that you will have sufficient light on the bottom of the tank directly under the lamps to grow most corals/clams. You will still have areas in the tank with lower light for placing corals that may not do well with high lighting. Note that the amount of light near the top of the tank will be significantly higher with the 400W lamps compared to the 150W lamps so you may need to be careful with what you place there.

Note that the above is simply a guideline and you could still do well with less light, but may have much slower growth rates for some corals.

There is also some relationship between lighting intensity and colouration of clams and pigmentation of corals. Higher light intensity definitely yields better colours in clams, however in the case of corals, higher lighting MAY yield better colours - no generalisation seems to apply and is specific to the individual colony.

What colour lighting should I get?

This is mainly a matter of preference. Some people prefer the "blue" colours in their tank, some prefer towards the whiter scale of things. 10,000 Kelvin seems to be the most common, and is pretty much a white colour. It gives a good balance between PAR and shorter wavelength light (short wavelength require more energy to produce). A bluer light will show more florescence in the corals. See Bulb Color Comparisons for images showing different combinations.

How much heat will the light generate?

All electric lighting work by converting electrical energy to light energy and heat energy. The more efficient the light technology used the more light is produced and less heat. The two common technologies used in the aquarium hobby today are Metal Halide (MH) and Fluorescent lighting. The efficiency of the two technologies are roughly equivalent, metal halide lighting producing slightly more lumens per watt than fluorescent. Regardless of the type of lighting used the same amount of heat will be produced. Deceptively MH lighting feel a lot hotter, in fact it is impossible to touch a lit MH lamps without causing serious injury. The reason they are hotter is the heat is dissipated over a very small area. For example a single 150 W MH lamp is approximately 132 mm long all the heat produce by this lamp will be dissipated along this length with a concentration of heat at the centre. To produce the same amount of light you would need 3 x 54W T5 lamps. Each 54W T5 lamps is 1163 mm long so same amount of heat will be dissipated along 3489 mm of lamp, the larger area means while the same amount of heat is being produced, it is still possible to safely touch the T5 lamp.

Greater lighting wattage will produce more heat, how this effects the aquarium is dependant on how well heat can be removed from the lighting fixture before it starts heating the tank. It is common to house aquarium lighting inside a hood, in this case if higher wattage lighting is used sufficient airflow should be provided through the hood, to remove as much heat as possible, this can be achieved through the use of computer fans or air extraction fans. In some cases if the room temperature gets too high additional tank cooling may be required.

How much will it cost me to run?

You can calculate your own lighting quite easily, but it will depend on the total lights that you have installed. All lighting per watt will cost little difference no matter what type, a 150w MH is no more expensive to run than 150w of T5 or 150W of fluorescent, its still 150w being used.

To calculate the cost use the following formula.

Total KW used per day = (Total control gear watts X Total Hours Per day used)X Hours used per day) / 1000 The just multiply the KW per day with the price per KW you pay eg $.12 eg ((250W x 10Hrs)/1000 = 2.5KW per day 2.5KW per day @ $.12 per KW = $.30 per day

Here are some typical control gear total wattages:

150w MH = 168w total

250w MH = 271w total

400w MH = 424w total

40w Fluro T8 = 49w total

54W T5 = 58w total

Does T5 lighting of the same wattage produce the same amount of light as metal halide?

On a lumens per watt basis, fluorescent lighting is 'roughly' equivalent to metal halide, with possibly a slight advantage to the metal halide.

There are a whole bunch of factors that can impact on both types, like:

  • lamp selection
  • reflector design
  • electronic ballasts
  • placement height above water

But both are in the same ballpark give or take 10-20% in terms of light output per watt of power consumption.

T5s can be fitted with a particularly efficient reflector, and can be placed very close to the water surface, which all acts in their favour.

The big difference between the two is light distribution. The metal halide light source is concentrated into a very small area and has much better "punch" to get to the bottom of the tank. There will be points directly under a metal halide lamp that will be MUCH brighter than under the T5s. But there will also be areas to the side of the metal halide lamp that will be less bright than the T5s.

In this perspective, metal halide gives much more flexibility in finding the right position for a particular coral. A 24" high tank (even with a sand bed) is starting to get fairly tall, and metal halide may well be a much better option if you are wanting to get light penetration right to the bottom of the tank.

Also, don't be fooled by the cost of the bulbs. T5s can cost more than it would cost for a higher wattage metal halide. And a metal halide lamp will last up to 24 months. Not really know how long the T5s will last yet, but unlikely if they go more than 12 months without significant loss of intensity.

Lighting Intensity Definitions

This is a very subjective area to define. There are many dependent factors that will affect the quality of light, including light source and reflector type. Regardless of water column depth lighting is always going to be more intense closer to the light source. So as a rough guide going per foot of water column depth:

  • Low Intensity
    • Less than 100W per foot of water column depth.
  • Medium Intensity
    • Greater than 100W but less than 200W per foot of water column
  • High Intensity
    • Greater than 200W per foot of water column