I would not use any antibiotic in your display aquarium. If a lionfish breaks its jaw then there often is nothing you can do outside of taking it to a qualified aquatic medicine specialist for surgery. Do NOT attempt to handle the fish yourself. There is a chance that the injury could heal, but it would take a long time so only a large fat lionfish could be able to survive without food that long. Feeding the fish before it is fully healed could result in re-injuring the jaw.