Plant looks 3-4 weeks old, that's the time calmag issues usually show up.
If you have some cal/mag then give a dose, follow instructions on bottle.
You can also give a foliar feed of calmag which hits it where it is needed and works quicker then just root feeding it.
I use Epsome salts as an alternative (I have hard water that sorts out my cal) 1 Tbsp in one gal/4.5Ltr of water or 1tsp in 2pts/1ltr of water.
I have a 1ltr spray bottle I use for foliar spray of epsome salts.
Epsome salts are crystals which need to be dissolved in warm water then topped up with cold water and then ph it.
That's just the way I do it and it works for me, someone else may do it a bit differently.
Your plant is wider than tall! My reasoning is you have a lot of light in your bucket 200W cfl & it's dual spectrum so plenty of blue in it and you have it only inches from the top of your plant.
Abundant light especially in the blue spectrum during the veg stage, will keep you plant stocky, I get this with my LED's, I have lots of light and lots of blue in when I select veg mode spectrum.
My plants look like mops until the reach stretch time, which your should be about to do.
Great little set up ,i have always liked the look of Space buckets but haven't built on yet, currently building a PC server grow.