I don't know that the spectrum of a T8 looks like - can you post a picture of the spectrum.
The spectrum of a 6500k light is shown here:
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Is blue heavy which is used for plants in the vegetative stage. Blue photons inhibit cell expansion so plants grown under a lot of blue light will tend to be short and compact. That's desirable in many situations but there's a "blue photon penalty" when it comes to yield. Typically, growers will use lights with a blue heavy spectrum in veg and then switch to a light that has a high percentage of red in the spectrum in flower.
Red photons do not inhibit cell expansion, the promote cell expansion and they're significantly more electrically efficient than diode that generate blue photons. That's where legacy growers used HPS lamps which have a high percentage of red.
The reason to ask for the spectrum of your light is that, per Bugbeee, grow lights should have at least 4% blue in their spectrum so as to avoid malformed plants. That number looks an awful lot like the percentage of blue that the Mars has in one of their new "greenhouse" lights.
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It would be surprising is the T8's don't have some blue in them. The 4% number can't be a trade secret so I'd have to ask - what else changed when you went from the T8 to the 6500k light?
Re. HPS to LED - the spectra are different, one difference being that LED's don't heat the plant and the grow space the way HPS does. I've seen research done that compares HPS to LED and there's no mention of any difference in nutrients so I suspect that it was an environmental issue (different temperature = different VPD = different transpiration rates = different nutrient uptake) that causes nutrient issues.
