wow....ghetto rigged fan there....

i assume that the Mh is for veg and not to flower, or youd want to consider an hps instead

it doent look like a light issue. howver, it may be a heat issue. how powerful is that bulb, cuz it looks within a foot of the plant?

besides that, it could potentially be minor nute burn or magnesium lack.

what ferts do you use?