Huh, that actually looks like drying from a combination of fan blowing right on a part of the plant, low humidity, and possibly the plant being too limited on root space to replace the water lost through transpiration.
6 feet is QUITE large indoors! What's up with that? In soil, that would require at least a 10 gallon pot to survive... in hydro, I'm not sure, but you're really pushing the limits for plant size indoors.