While replacing the light tubes in one this morning (the kind of tubes with 2 prongs that roll into place), I broke off a small piece of plastic in one of the fixture 'sockets' that helps hold the prongs in place. The opposite socket isn't broken and is holding well enough that the light still works, but I'm concerned this isn't a long-term solution. Can those sockets be replaced individually or does the whole fixture need to be replaced? TIA to the experts!