T12 or T8, with magnetic ballasts, fixtures are unable to start at low temperatures. The lowest temperature where they start dependably is around 55-60F. Could it be that cold where the aquarium is? If not, I don't think it will be the starters that are faulty, since having two starters fail at the same time is unusual. When I had T8 fluorescent shop lights in my garage a few years ago I could never get them to come on in the winter, because of the cold start problem.