In part that, and in part because nobody makes a cost-effective 680nm LED... 660nm is as close as it gets, and is still well used by plants. The next commonly available step up is 730nm.Ah, so it's going by chlorophyll absorbance and not photo-systems. Interesting, thanks.
Based on the "action spectrum"..So I was sold on the fugeray planted+ because I study biology and was fascinated with their "true red 660nm- which studies have found to be one of the most efficient wavelengths for plant photosynthesis" LEDs. As I recall however, Photosystem I functions optimally at 700nm and photosystem II functions optimally at 680nm. So where is this 660nm spectrum coming from? I would really like to know the methodology and research supporting this optimal red spectrum claim.
At typical aquarium depths of 10-24"? Not likely... Even at 700nm and a full meter of water depth you're around 60% absorption in pure water.Honestly water absorbs red light so effectively that above 660nm you are probably losing 75% of the PAR to the water anyways.