dark brown spots on my banana tree
 

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

My guess is fungus and it can be caused by poor circulation. Add a fan to simulate the wind blowing on your plant.
LOGICAL PROGRESSION OF FUNGUS CONTROL
1. Prevention
a. cleanliness
b. low humidity
c. ventilation
2. Remove infected foliage and plants
3. Use copper, lime, sulfur or potassium based sprays for bad infestation.