You are saying your light fixture is 12" long and the tank is 30" long and the light from the fixture doesn't cover the entire tank, is that correct?
Having dark ends isn't bad, but don't expect much to grow in these darker areas. Most black boxes are 16" long and cover roughly 2 feet of tank. If you want to light the entire tank, you will need an additional fixture or a new one that is longer. Both leds and t5 fluorescent fixtures of 32" and 48" would do the job.