I'm still not sold that light and color temp doesn't make a difference....sell me.

Well, the temp or Kelvin rating is not the same as intensity.....

Are you running say 6500k on one tank and 14k on another?

 

By doubling the amount of light, plants usual not grow twice as fast but more like 50% faster. This is because they are not able to process all the light and in a response they turn red, so they no longer receive light from the red spectrum. Now they only receive blue light because chlorophyll is only sensitive for blue and red light, green light is neglected in green and red plants.

Wavelengths chlorophyll is sensitive to:


More light does make plants redder. About color temp (kelvin) plants don't care. Plants want PAR and this usual isn't cited on the package, so bulb A 6500K can have double the PAR output compared to bulb B. When bulbs have the same PAR output but different kelvin ratings like 3000K vs 14.000K (yellow/red vs blue), the only difference I (and others) noticed is you get more compacted 'bushier' growth with high kelvin and plants reach more to the light and become 'leggy' with red/yellow light. Hopefully this is useful for you!

 

This is misunderstood a lot. Plants that colour red is more a syndrome than a virtue of health. That's why I keep the light relatively low (80 mmol). Good growth, health, strong plants, no stunting and no problems with algae. Yes, my green plants stay green. Too bad that is mostly seen as a shortcoming.

I've tried various bulbs between 3000 and 10000K. I can't detect any mentionable difference in growth, only in colour because of the difference in reds and blues from the bulb.

 

Something changed when I swapped in 12 warm white xmls for 12 cool whites in my led fixture. I have 28 XML total, and used to run 100% cool white. The colour was very crisp before, like a very clear day with no clouds or fog. The light itself was beautiful, but the only thing that looked good under it was green. The plants looked washed out and only a few colours looked good. Purple and pink were kinda nice looking, but red was missing.

Anyway, the look is much more yellow now, but the plants and fish look much better with the warm whites. I have Ludwigia repens 'rubin' that has shown the biggest difference. It had a smidge of colour, but since the change it has been taking on a bronzy red colour. When I first got my Hygrophila 'bold' it was a nice rich green with purplish tones to it. Very prehistoric looking. Under my light the green stayed nice, but the purple hue faded. It's coming back now. My Pogostemon stellatus was completely green before the warm whites, even the underside of the leaves. They are slowly colouring up now and are showing some pink around the nodes.

The only thing I've changed was swapping in the warm whites. Co2, par levels were kept the same, although the warm whites may register different to the par meter. I did however find I was getting a bit of red before the LED swap (although it looked washed out) so around the time of the swap I upped my dosing to insure I wasn't accidentally limiting nitrates and forcing the reds. In my opinion this strengthens the hypothesis that the warm whites should get the credit for the better colouration, because if the nitrates were limiting before, they would be less so now. And the colouration is better/ getting better.

The day I swapped the LEDs there was a noticeable difference in the look of the plants. They appeared to be different colours instantly, although the effect was subtle. In the week since, the colouration of a few plants has changed drastically. Mostly on new growth, but also on old growth, like the Ludwigia. I trimmed the stellata (I have to trim it every couple days) and so the new, more colourful growth is appearing further from the lights. So I can't attribute the colour change in that particular species from being closer to the light.

My LEDs are 20" above the water and 40" off the substrate. The par drops by around half from top to bottom if you are directly under the lights. Around 140-170umol at the surface and anywhere from 20-80umol on the soil. I'm just adding this to point out that I don't have the extreme increase in par as you get nearer to the top of the tank like many people have.

This photo is with all cool white xml (for main power, I have rb and nw xpg for colour temp adjustment which add little to no par). The photo looks as accurate to real life as I am capable of.


This next pic was taken the day I swapped in the warm whites. It's an iphone pic and it is a horrible pic, but you can see some differences. I had also trimmed. The pic is unfortunately blurry and overexposed.