Furan is an antibiotic. It won't harm the shark but the shark will build up resistance to it. However, the disease that is in the tank may be picked up by the shark in its weakened condition.
Two sharks in a tank is a no no. Mass sharks will spread the aggression. Being chased is not the issue here - that is definitely worms, I have seen it many times before.
They mostly clean up leftover food so while flake will work, there would need to have been enough left over. They also graze on algae. Bloodworms if feed should be in small quantities (like 2-3) at any one time.