GBA among other types of macro and micro algae can grow in otherwise healthy aquarium water, since they have the ability to photosynthesize a portion of energy required to grow. Knowing that, in addition to knowing that the algae's most active photosynthetic absorption peak is right at 640-670nm (red colored light), reducing the levels of red light in conjunction with maintaining proper water quality is like a one-two punch to algae.
If you are feeling nerdy, here's a bit of background on how this works:
T5 bulbs work using a mercury based gas, when high voltage is applied, it glows blue/purple. Using the white powdery phosphor coating on the inside of the glass reacts with the light emitted by the mercury gas creating a white (or blue, purple, pink, etc) light, which is what we use for our aquariums. (Phosophor the same material used for glow-in-the-dark toys, etc). The glow from phosphor is called florescence, hence, this is the reason the tube bulbs are called fluorescent lights! Unfortunately the phosphor in fluorescent lights [ie T5 bulbs] degrades over time, and emits less of the beneficial shorter wavelengths of light (bluer) and shifts towards and emits more of the longer wavelengths (red-er) light. That said, the specs on your T5 bulb are only valid for about 2 weeks until the light output begins to shift ever so slightly towards the red spectrum. After 7-9 months the shift is so huge that a big amount of blue light, as shifted (wavelength increase as the phosphor degrades) to the longer wavelength red light, creating issues with providing enough essential wavelengths to both grow coral and reduce algae growth. If you've talked with researcher in this field studying the effects of red light in marine aquariums, any red light in an aquarium is usually referenced as a "toxic color." They call it toxic because it produces annoying, unpleasant, non-beneficial, and sometimes harmful cyanoalgae and similar micro-algae.
If you are following along so far, you'll ask me about the phosphor used in LEDs, and how is it better or how it differs from T5 (and other florescent lights) right? Well the phosphor used in LEDs is a similar type of phosphor, yet does not degrade as fast. Why does it degrade at a infinitely slower rate? Because the blue LED that illuminates the phosphor has a very specific and non-harmful wavelength of about 440nm. Blue light emitted by mercury vapor is about 280nm-410nm - in the harmful ultra violet range. Ultraviolet is a very high energy wavelength that degrades all type of material, just like sunlight does. It will degrade paper, human skin (sunburn), all organic DNA, plastics, retinas in eyeballs, corals, anemones, clams and even phosphors. Basically the LED does not output ANY ultraviolet light to degrade the phosphor, and the insignificant degradation that does occur, is due to heat. That said, regarding the phosphor used in LEDs, the degradation period is on on the scale of decades, not months. The color shift of an LED after 10 years of 12/hr/day operation, will have shifted in such a small amount that it is considered negligible compared to a florescent phosphor. I suppose this is just another reason the trend towards LEDs is increasing in popularity, because the LEDs have this advantage to output little to no red light, and completely curb algae growth, despite water conditions.
Lack of red light curbing algae growth is scientifically proven (even with a surplus of phosphates in the water), also evident by a complete lack of algae growth at deeper ocean water levels, where red light is unable to penetrate.
Having been successfully utilizing LEDs in aquariums for more than 10 years now, there are definitely some LEDs out there that output a very heavy amount of the red wavelengths. At Steve's LEDs, we make a SIGNIFICANT effort to engineer our LED systems to minimize the amount of red light emitted. Using a tool called a radiospectrometer allows us to see the wavelength intensity of the light output emitted at the 1nm resolution, so we can pick and choose exactly what corals need and avoid what algae requires to grow. This reduces algae growth as much as possible, yet does not impede coral, anemone, or clam growth in any way. There is not way to completely eliminate red light and still have excellent aesthetics - eliminating red light results in a perfectly blue/green aquarium and washing out every other color- not very desirable. That said, we have only the minimum amount of red light necessary to get the corals and other inverts looking amazing, and this such a low amount that algae is not able to photosynthesize it.
Those companies using red LEDs in their fixture are either not actually aquarists, or they are trying to make their own "fad" LED combination, or perhaps they are valuing the numbers on their spec sheet vs actually caring about growing corals as healthily as possible. Steve's LEDs considers all new "fad" LED combination that pop up, however, we take great pride in only putting into production those LED combinations that are scientifically and research proven to appropriately balance invert growth rates and aesthetics, with a huge emphasis on algae reduction.
Hope this helps 'enlighten' you on understanding algae growth in your marine aquariums.
Jeff