The 5100k is within the color range for plants-Ideally you want 6500k-Some species of plants will grow in the color range of 5000k-10,000k.
With florescent bulbs you also need to change them about every 12 months-due to the intensity can be lost/used up over time. We can see the light-but the plants can't....
The yellowing or die off can either be a normal die off or nutrient/light related. Pinching the ends or dead parts-like what you are doing is what you want to do-you might want get a 6500k when it is time to change your bulb or if the plants start to look worse. Also, you might want to add some ferts if you don't already and follow a photoperiod of at least 10h/day if you are not already. I stay on a 12h/day in the warmer months and drop to 10h/day in the cooler months.
Last edited by Rainbo; 03-31-2019 at 01:08 PM.