OK, so these AK-48 plants where sprouted almost 4 weeks ago. There are five plants, which I started under four 4′ 40w fluorescent tubes. When it was evident that the plants were stretching badly (growing 4" in a day and falling over), I added two additional 150w HPS bulbs and a small fan. This seemed to solve that problem.
I transplanted them from their starting cups to 5 gallon buckets about a week ago. The soil is all Miracle-Gro moisture control potting soil. Except for the stretching, they have seemed extremely healthy until now.
As you can see in the pictures, the edges and tips of some of the leaves have begun to yellow. The initial two sprout leaves have completely shriveled, though I assume this is normal. On all of the plants the single bottom leaf has yellowed at the tip (second and third picture). Additionally, on a couple of plants the first real (bottom-most) fan leaves have yellowed slightly at the edge (first and fourth pic).
I read that this type of yellowing was sometimes a symptom of Nitrogen deficiency, especially because it was starting at the bottom and going up. So, two days ago I fed the plants with fertilizer for the first time with Miracle-Gro tomato fertilizer. Two days later the plants look the same: no better, no worse, and still growing steadily. The rest of the leaves are a healthy looking deep green.
Some additional information: I have been giving them a decent amount of water every 3-4 days. The water is filtered city water, which is then left out for several days. Also, the room gets a bit warm when we have hot days, but nothing over 90 degrees or so on the worst days. A fan keeps fresh air circulating from outside.
So, am I worrying over nothing or is there something I should be doing?