In order to begin to understand the available spectrums of bulbs, we must first analyze how light naturally penetrates water. Red light is the first to be filtered out and can only penetrate a short distance. As light waves penetrate deeper into the water, orange and yellow are lost next. Of all the colours of the spectrum blue light penetrates the deepest.
Light spectrum is measured by the Kelvin scale. Natural sunlight on a clear day registers at 5500 Kelvin degrees. Kelvin temperatures less than 5500 become more red and yellow and the higher the Kelvin temperature the more blue the light is. Photosynthetic invertebrates should be kept under lamps rated at or near the Kelvin temperature where the invertebrate was collected. Shallow water species should be kept under 10000K lamps while deep water species would prefer 20000K lighting. Longer photoperiods cannot compensate for incorrect light spectrum or intensity.
Actinic lighting peaks in the 420 nanometer range and emits a fluorescent blue light and is usually used as supplemental lighting. Not only is actinic lighting beneficial to photosynthetic invertebrates, it is also aesthetically pleasing to the eye when used to supplement "daylight" lighting.
Freshwater aquarium plants benefit from lighting with a Kelvin temperature in the range of 5500 - 6500 degrees. Freshwater plants prefer light with more red and yellow in the spectrum.
The context with which I believe the statement that actinics are for nothing more than aesthetics is that it isn't that important what the composition of the spectrum of the bulb is, as long as there is enough light of the appropriate wavelengths for photosynthesis to occur. So as long as you satisfy the needs of photosynthesis, beyond that it doesn't matter what bulbs you use, actinic, day light etc.
The spectrum from actinic lights are important, because of the fact it is important for photosynthesis. So on this basis saying actinic lights are for aesthetics is incorrect, it can be very important if it is the important/dominant source of light of the right spectrum for photosynthesis to occur.
Whether actinics is important depends on how the tank is set up and the amount of light that the actinics contribute to the total photosynthetic available radiation. For example if you are running a 250W 6,000K MH with 40W of NO, then the NO contribution to the PAR is pretty insignificant and whether you use actinic or daylight is of no real consequence to the photosynthetic organisms in the tank. But your choice of NO will make a difference in how the tank appears to the human eye. Now what happens when you turn around and are just running NO or VHO over a tank? Half daylight and half actinic, are the actinics now there just for the look of the tank? Of course not, now it is making up a significant amount of the PAR.
So to answer your question a bit more directly, yes actinic may aid photosynthesis in your freshwater tank. However, it depends on what other light sources you have available and what their spectrum range is. It also depends on what type of plants you are trying to grow. If the plants are not normally deep water plants and you have other full spectrum bulbs, or grow bulbs, on your tank it is unlikely that actinic lighting would have much effect on your tanks photosynthetic potential. If you are growing algae the actinics could be more important. Generally I associate actinics with salt water tanks and corals. The blue colour could be very pleasing in a fresh water tank though.
What Is Actinic Lighting?
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14 August 2011
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