Honestly i would keep it on as much as possible with out growing to much algae .i had the same exact problem with my community tang tank fish were getting real beat up sometimes killed by the dominant fish when the lights went out . So i keeped the light on at night but then had an algae problem. Eventually i had to change some of the stock in that tank now everthing is good. But for the long run its best to remove one of the fish because your gunna end up with one of the two problems in the future . Or maybe re scape your tank add more sight barriers.
Good luck
Dan