dark brown spots on my banana tree

[Gerrit]

Can you explain the dark brown spots I am seeing on the tops of the leaves of my banana tree? I have the plant in a two bucket (aquafarm) hydroponic system. The media is grorox and the light is supplied by a 1000W MH. The leaves are at least 4 feet away from the bulb. I just changed the nutrient solution in hopes that this would solve my problem, but so far I see no changes. The plant seems to be doing fine, but these spots and blotches make me a bit uneasy.
Thanks,
Gerrit