The two sides of the red plant problem are:
Does the light emit enough light in the red part of the spectrum for you to see reflected red light?
Does the plant need red light in order to turn red?
I don't know the answer to either question for sure, but I don't think plants need red light to turn red, just enough PAR. And the first question would have an answer dependent on what LED light you are referring to.
I wholeheartedly agree with both points. Red plants do have some special needs but not in the ways most think. Water hardness is another lesser discussed factor, worth having a read... :
it goes along way to understand the metabolism of the plant and while having fast growth is nice, some varieties of the plant may not have sufficient time to develop in the ways to look its best.
To some degree i agree with the PAR being an important first guideline.
I can tell you from experience the Ray2 will not grow red plants other than those that are "red by default" and do not have a true green form. Plants like ludwigia and Rotala will eventually go green under the Ray2.
Everyone likes to quickly answer "you need to dose iron", or "you need to limit nitrates", or "you need an iron rich substrate" and technically all those things will promote nice vibrant reds in your red plants, but if your lighting is not producing enough red spectrum light, it won't matter how much iron you dose or how great your substrate is.
Red plants (speaking for red plants that also have a true green form) will only grow red with sufficient red spectrum lighting. It's a defense mechanism. Red plants do not like light in the red spectrum, therefore they turn red in order to reflect that light. If there is no red light, there is no need for them to defend against it and they grow green.
I just ordered a dual T5HO yesterday to replace my Ray2 for this very reason. I can't keep my reds red.
Here is the Ray2's spectrum chart, as you can see, there is almost no red light:
To say LEDs cannot produce red plants is also incorrect, but most commercial fixtures currently available to the hobby simply don't offer enough light in the red spectrum. I have seen DIY LED fixtures that can produce red plants, so you could always go that route.
I am by no means an expert on lighting, but I did my research to figure out why my plants were going green and this is the conclusion I've come to.
I am also not one to assume the lack of nutrients dosing bandwagon as this is contrary to my own experience. I've done a fair bit of testing varying the color spectrum in a few tanks with some interesting results, mainly with RGB adjustable LED lamps. Firstly, they are noticeably dimmer than my RAY2s but the color adjustments are infinite. This sort of test is only possible if the comparison is done across multiple tanks with only a single variable changed.
The successful aqua gardeners that i know commonly rely on T5HO lamps (2,4,6,8 bulb configurations) with all using a variety of bulb types, with some even simulating dawn and dusk photo periods. It would be naive to think that a single LED fixture with a fixed color could come close to what a fluorescent offers. If LEDs were the way to go, RGBs or multiple fixtures at varying Kevin diodes would be a step in the right direction. I felt this point needed to be made so apologies for being long winded.
I ended up buying the AquaticLife dual T5HO for $115 on Amazon. I actually just hung it about 20 minutes ago. It doesn't have the same crisp clean brightness the Ray2 offered, but just by looking at it, you can see there's more red spectrum. It comes with one 6000k daylight bulb and one 650nm roseate bulb. And I can always change out bulbs to meet my needs.
If you can find a cost effective LED solution, give it a shot, just make sure the spectrum of the light is 630-670nm. 650-660nm is best from what I've read.
Agreed as I also am contemplating adding a single T5HO strip to supplement my RAY2 or even downgrading to Fugeray + T5HO supplemental lamp and not overwhelm the tank with light.
The light light has been up for a month now and guess what?
All my red plants are turning green. Except for one and. Also I had trouble with a few melting I guess they where to delicate to handle the shipping. But they are slowly coming back. I would love to get these reds popping again.
To some degree, we need to look at serving a variety of color frequencies in the tank. Just as EI dosing covers all the nutrient needs of a plant, this same concept should also be applied in full spectrum lighting to serve the needs of all plants.