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The Five Most Important Criteria for Selecting Aquarium Lighting

In this article you'll learn about the different criteria to consider when selecting aquarium lighting for your tank. Watts per gallon was long considered the hard and fast rule. Not any more. Read on to see why.

When it comes to selecting the right kind of lighting for your particular aquarium, you need to consider more than just the watts per gallon. Though the number of watts per gallon is a valid piece of information to consider, it is not the most important. Back when T12 lights were standard for most aquariums, a rule was developed stating that 2 to 5 watts of light per gallon was adequate. Because those T12 lights are no longer standard – in fact there are many different modern lighting systems to consider – this rule no longer applies. Today, the five most important criteria to consider when selecting aquarium lighting are PAR, PUR/Useful Light Energy, lumens per watt, lumen focus and watts.

PAR (Photosynthetically Active Radiation)

cichlids xs 3801341Photosynthetically Active Radiation (PAR) is simply light that falls within the 400 to 700nm range. This is the type of light that live plants and photosynthetic invertebrates like coral need in order to thrive. Aquarium lights that are actinic UVA and infrared are the most likely to produce light in the PAR range. UVA light falls within the 400 to 550nm range and infrared light falls within the 620 to 720nm range – thus, the best PAR output will occur when a balance of UVA and infrared light is achieved. Daylight bulbs of around 6,500K have been shown to produce the best PAR output. Achieving the right PAR output is important for the health of all tank inhabitants, not just live plants. Studies have shown that fish kept in tanks with full spectrum lighting (which tends to produce a greater PAR output) had better immune function and were more resistant to disease.

PUR/Useful Light Energy

discus 000001915432xsmallPhotosynthetically Useful Radiation (PUR) is also referred to as Useful Light Energy and it should be one of the major criteria in your selection of aquarium lighting. Whereas PAR refers to any light between the 400 and 700nm range, PUR spikes at wavelengths between 400-550nm and between 620-740nm. PUR is simply the usable portion of PAR and each photosynthetic organism responds to a different PUR range of light. There is no good way to measure PUR, though spectrographs and other pieces of modern technology can be used to measure the nanometer spectrums of aquarium light. Spectrographs have limited use when applied to PUR because they can only measure the spectrum of light in the air – PUR is affected by the depth of the water in your tank. When it comes to choosing aquarium lighting, you need to know what kind of tank inhabitants you plan to raise in order to select a bulb with the proper PUR rating. For example, if you plan to keep photosynthetic invertebrates in your tank which require light from the blue end of the spectrum (in the 440 to 490nm range), a bulb producing energy that peaks at 490nm would have greater PUR than a bulb producing energy which peaks at 590nm.

Lumens per Watt

light xs 21606922The lumen is the international unit of luminous flux and it is used to measure the total amount of visible light emitted. In simple terms, the brighter or more intense the light, the higher the lumens are likely to be. To determine how many lumens per watt any given bulb provides, simply divide the total number of lumens by the wattage of the bulb. Though knowing the number of watts per gallon a bulb provides was once considered the most important factor in choosing aquarium lighting, the number of lumens per watt is currently considered more important. A watt is a measure of energy, specifically a measure of the energy used by the light fixture rather than the amount of light produced by the bulb – for this reason, the lumens per watt produced by any given bulb is a more useful calculation. For example, a 15-watt bulb rated with a total lumen output of 800 lumens would have an output of about 53 lumens per watt. A 10-watt bulb with a total lumen output of 950, however, would produce about 95 lumens per watt. In this example, the bulb with the lower wattage would actually be more beneficial for an aquarium.

Lumen Focus

lights led xs 11281683Though the number of lumens per watt is a useful calculation, it can sometimes be deceiving depending on the type of lighting to which it applies. LED lights, for example, use focused light energy and thus require fewer lumens to produce the same intensity of light. If you were to compare an LED light fixture rated at 400 lumens, you might be tempted to select another kind of bulb because it has a higher lumen rating. In reality, that LED fixture could actually produce a higher intensity of light despite using as little as half the lumens of other fixtures.
Watts per Gallon

One watt is equal to one joule of energy per second and it is the unit of energy used to describe the amount of energy used by any given light fixture. At one time it was common to compare aquarium light bulbs based on the number of watts per gallon each bulb could provide – today, other criteria are considered to be more important. It is still useful, however, to know how many watts per gallon a bulb produces, as long as you know how to use the information properly. In order to use the number of watts per gallon to compare two bulbs, those bulbs must be the same type. The 2 to 5 watts per gallon rule can only be used when making “apples to apples” comparisons, not when comparing one type of light fixture such as LED or T5 to a completely different fixture like metal halide or VHO. Overall, though it may be helpful to know how many watts per gallon a bulb produces so you can determine whether it is enough to meet the requirements of aquarium plants, this information is best used in conjunction with the other four criteria for selecting aquarium lighting.

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