My main suspect is this: lack of humidity ???
I say for two reasons:
-I've never heard of a disease causing leaves to fall without turning a yellowy-brown first
-The tree is still growing beautifully
I water my tree only when the upper layer of the soil seems to start to dry, and I keep my fluorescent light lit for around 16 hours a day, when I'm not sleeping. I mist 1x per day. However, the tree is placed about 5 feet from a hot air vent, which is probably causing fairly dry conditions for the tree. I've just filled my drip pan with water as this may help stabilize the humidity.
The only other problem I can think of is my fertilization routine. I add a label-directed amount of 10-15-10 liquid fertilizer to my water every other week.
Can anyone help me? The tree is still very full looking right now, but I'd really rather it didn't go completely bald

Thanks for looking!