Well, I've been testing a lot with the use of LED-only lighting above my main planted freshwater tank recently, and noticed that the plants really require more of the low blue than the red. Without it they simply die off.
It's still quite hard to find decent LED-bars or strips to use for this purpose. You can only buy panels (30x30cm and such) or Tube-replacements (T8/T5) and all are extremely expensive per lumen. I just refuse to go for those commercial solutions. Usually their packaging is already crap to begin with. I don't need 24V, 110V AC or 220V AC, I just want 12 VDC.
Turns out all the AC-powered LED-tubes/panels/grow-light blink! And not a little, even (
see here in Dutch).
I had already skipped PWM dimming (to mimic sunrise and sundown with) and went for a timer circuit using a sequence of on and off 'channels'. So I sure don't want blinking LED for my biotopes. The sun doesn't blink either..
OK, so I found out the most effective (Lumen/Watts/photosynthesis) ratio for freshwater underwater plants is around 5:2, where 5 consists of dark blue peaking around 428 nm (for most plants) and 2 is deep red peaking around 664 nm.
If we would have, say, a flexible or rigid LED-strip, waterproof (at least IP65) similar to
these types, they would be perfect with LEDs at those frequencies. Nobody seems to sell or even make those LEDs, which is strange, considering their huge market potential.
I have yet to find a decent LED frequency response spectrum for aquarium lighting that is commercially available now. Of course you can come close by using the lowest blue and highest red LEDs and use many cheap ones, but the thing is, as was already mentioned above:
You do not want the red-blue (i.e. purple) color to overrule all the lighting in your aquarium. Which is why you want perfect LEDs peaking at 428 nm and 664 nm so you can use the minimum amount possible. Saves you a lot of power too.
In my research I have not found suitable LEDs yet. Preferably plug-and-play modules, or at least pre-wired LEDs for 12 VDC. Most annnoying is the fact that very few sellers of LED-products mention anything about the frequencies of the LEDs used, let alone show a spectrogram. About time those specs become more common on websites (and ebay!).