You've looked it up and you found all the possible answers to your question! Now, you can figure out which one (or which combination) is the RIGHT answer if you really try and if you know your plant.
I would say, first of all, that being 6" away from their light source is a bit of a clue... back them off to about 18-24"!!
I don't think it's nitrogen... all of the leaves would be reacting and the plant doesn't look, in general, like it's getting too much of that nutrient.
Temperature can be a result of being too close to the light source... but if you don't have a fan going, that would help.
Now comes overwatering (but you DID miss underwatering as a possible answer)... are you letting the pot dry out sufficiently between waterings/nutes? Do you lift the pot to see how light it is before giving it something or are you on a rigid schedule "Oh, it's watering day, I MUST water the plant" kind of thing... Do the former, not the latter...
If you know the possible answers (and you do), and you know what you've been doing to your plant and the conditions it's living in (and you do), you can figure it all out! Trust yourself!!
Good luck...