Which nutrient or nutrients are most responsible for thick, healthy stems?

My leaves are fine, my buds are fine, but my stems are consistently puny. It hasn't been a hinderance exactly, but I am somewhat concerned about how skinny my stems are.

Rundown:
Fertilizer: General Organics GoBox supplemented with liquid silica
Soil: Hand mixed Organic
Lighting: 900 watts worth of LEDs, two T5 UV-B bulbs
Veg lighting: 24/0 Temperatures: Around 80° give or take 5° depending on ambient Container: 36"x36"x80" grow tent
Fans: Two 6" clip on fans elevated over the tops of the plants, pointed downward

I don't know if the temperatures are causing stretching and therefore artificially thinning the stems, or if I'm lacking something in my nutrient profile that is limiting their growth. I'm hoping it's the latter, because really 80° is about as low as I can go without turning lights off.

Is there anything you can suggest that might help beef my girls up? Anything that I should add, subtract, multiply, or divide to strengthen my girls? As I said, aside from skinny stems the ladies are pretty damn healthy.

Thanks in advance for any help or insight you can offer!