Thanks to everyone for the advice. Since the fixture came used with the tank, I didn't have a URL handy for the model, but I do have new info.
My husband got it apart, and the ballast is toast. A resistor blew right off the board. Of the resistor, ballast, and bulb, we're not sure which is the chicken and which is the egg, but it's definitely done for.
Thanks again for your time, guys. Off to search the boards for a replacement.