In my experience of it was a ballast you would get some type of electrical odor. Are the sides of the bulb dark in color or even black? Might just need a new bulb. Does it flicker? I'm gonna say bulb.
Have you checked to makes sure its in correctly? Make sure it clicks when your turn it. Just another thought.
"When creating an aqua scape, strive for the beauty of imperfection, as its art and there is no wrong or right, it's left open for interpretation" James Findley