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Why It's Important to Consider More Than Watts per Gallon

When selecting an aquarium lighting system for your freshwater tank, you may be advised to make a decision based on the "2 to 5 watts per gallon" rule. While this may be a good starting point, it may not always be accurate considering the many choices of the latest modern lighting systems.

aquarium xs 6119838When selecting an aquarium lighting system for your freshwater tank, you may be advised to make a decision based on the number of watts per gallon a certain bulb provides. While achieving the recommended watts per gallon is an important part of maintaining a healthy tank, it is not the only thing you need to consider when choosing aquarium lights. The fact is that the 2 to 5 watts per gallon rule is a guideline to get you started – modern lighting systems put out varying lumens per watt, so this rule may not always be accurate.

Limitations of the Rule

light xs 16465665A watt is a measurement of energy, not the amount of light produced by any given light bulb. One watt is equal to approximately one joule of energy per second – therefore, the wattage of a bulb actually measures the amount of energy a light fixture uses, not the amount of light it produces. Lumens, the international unit of luminous flux, are used to measure the total amount of visible light produced by a bulb. The higher the number of lumens a bulb produces, the more intense the light appears to the naked eye. While knowing the number of watts per gallon produced by a light bulb is a useful starting point, knowing the number of lumens per watt is more useful. For example, a 20-watt bulb rated with a total lumen output of 800 lumens would have an output of 40 lumens per watt. A 13-watt bulb with a total lumen output of 950, however, would produce about 73 lumens per watt. In this example, the bulb with the lower wattage would actually be more beneficial for an aquarium.

Key Considerations for Aquarium Lighting

The average wattage per gallon rule was devised back when T12 lighting was standard for most aquariums – there are so many different types of lighting currently available that this rule is no longer enough to make an informed decision. The most important criteria to consider when selecting an aquarium lighting system are PUR/useful light energy, PAR, lumens per watt, lumen focus and watts. Look for an explanation of these criteria in a separate article, titled “The Five Most Important Criteria for Selecting Aquarium Lighting.”  In addition to these criteria, it can also be helpful to consider the kelvin rating, the spectrum and the lux of any given light bulb. Before you can evaluate an aquarium lighting system to determine whether it is suitable for your tank, you need to cultivate a basic understanding of these terms so you know what it means when you find a 6,500K Full Spectrum Daylight bulb.

Additional Criteria Explained

You may be familiar with the Kelvin temperature scale, but this is not the same as the Kelvin rating used to describe aquarium lights. When it comes to aquarium lighting, the term Kelvin is used to define the color temperature of any given bulb. Lamps producing higher color temperature, those having measurements above 5500K, are considered “cool” and produce green-blue light. Bulbs which produce low color temperature, having measurements below 3000K, are considered “warm,” producing yellow-red light. The Kelvin rating is particularly important to consider for planted tanks – bulbs with a 6500K rating have consistently produced good results in promoting healthy aquarium plant growth.

uv lamp xs 7551264The spectrum of light produced by a light bulb is measured on the nanometer range. This scale is used to measure the wavelength of the light energy produced by the bulb and these measurements are taken in nanometers. The wavelength of light refers to the ability of that light to travel through aquarium water – red light (in the 620nm to 780nm range) can only penetrate short distances and is thus not recommended for deep tanks. Blue light (in the 440nm to 490nm range) penetrates the deepest and is the most beneficial for photosynthetic organisms because it is closer to natural sunlight. Many standard fluorescent and incandescent bulbs produce light from the yellow and green end of the spectrum – because freshwater plants cannot use this type of light, most of the energy from these bulbs ends up being wasted. LED, metal halide and other full-spectrum lighting systems waste less energy and produce lighting that plants and photosynthetic invertebrates can use more easily.

Lux is a measurement of the intensity of light produced by a light bulb and one lux is equal to one lumen per square meter. Though lux readings can only report the intensity of green light, the color to which the naked eye is most sensitive, it can still be a useful application when choosing aquarium lights, particularly for saltwater aquariums. If the lux is too low, the zooxanthellae inside corals will be unable to produce enough oxygen. Thus, the minimum intensity recommended for saltwater aquarium lights is 3,000 lux, measured at the deepest part of the tank.

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