Isn't the 6500k the temp of the colour that's best for plants to absorb though? God light can be so confusing. Isn't that why t5 is better than t8 for plants? Because the plants absorb the light better?
Light can be confusing. That's want the manufacturers and developers want. That's why they like to add labels and tag lines to their boxes that can trigger people in to buying their bulb over someone else's.
Sometimes you get a fancy graph or a CRI value as well as Kelvins, watts, sometimes PAR now it's clear we are moving more in that direction. Even with a PAR value at hand what can we really do with that? With 100s of different species of plants available to us we are never going to get a perfect PAR value. All tanks are different.
Kelvin is colour temperature which Essentially means the colour of black body that has been heated to varying temperature values. You might notice that a flame goes orange, red then blue then white the hotter it gets. This is why blue colour lights are often in the higher kelvin ratings.
The fact is that the bulbs we use have all the colours the plants need to grow healthily. Whether you use a 10,000k or 6500k. You can manufacture varying 6500k bulbs with a different colour mixes.
6500k also happens to be the colour temperature of the sun which makes people feel better I think.
ADA used bulbs high in the green spectrum for aesthetics but grew plants fine.
6500k tends to have a nice warm colour that looks more pleasing to the human eye when it comes to aquascaping.
As long as we are using bulbs that give out wavelengths in the 400-700nm range then the plants will use that light.
Perhaps certain plant allocation and morphology relies on certain colours but whether we use an osram fluorescent from the local hardware store or a bulb marketed towards growing plants they will both do that job well. The bulb marketed towards growing plants may reflect more colour that we find more appealing whereas the shop light may appear more yellow.
This is why I don't get too hung up on light. Keep it simple. Keep it generic. Better to test for ourselves. I've used all kinds of tubes and have had good success at growing both algae and plants. Just my two cents [emoji846]