Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
'k' is just what the light looks like to our eyes. It is a blend of colors, and often they are not the same colors that plants use. I'll bet you have lower light than you think you have.
Look into the better bulbs that are designed for plants. Look for the label that is like a little graph with several lines, probably filled in.
The peaks need to be in the areas where plants use the most. There is one peak in the red area, and one in the blue area.
Humans do not see the red and blue so well, so these colors are often missing or not very strong in bulbs for home or office.
Humans see yellows and greens best, but plants do not use these very much.
A light intended for humans is not usually the best for plants, and certainly getting several of the same mistake will not fix the issue.
If all you have to go by is the name of the bulb, and a k number then get a mix of bulbs such as Daylight, Cool White, or Plant and Aquarium. I mix 2 or more of these on my tanks and it is still not enough. The tanks are also near windows, and that helps a lot. any k value in the 6,000 will probably be OK, but get a mix. 10,000k is usually too far into the blue for plants.