Call the company they would be the best people to help you or look at the directions see if there are schematics of the fixture.
Are you sure it's the ballast? Does it smell? A bad or burnt out ballast sometimes will give off a very distinct odor of electrical burning. It may also by discolored.
I think starting with the company would be the best place to start. They probably offer up replacement parts also. Then see what ballast (of that's the issue) they recommend and see if you can get it elsewhere.
"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