Your molly fish is aggressive because it’s trying to mate. The mating rituals of molly fish involve attention-seeking behavior, which usually translates to a male molly fish chasing a female around the tank. This behavior will usually stop after the fish are done mating.
Additionally, like other species, male molly fish can sometimes be aggressive towards other males. This usually happens when they’re fighting for mates or when they’re defending their territories.
Molly fish are generally peaceful, but some bouts of aggression can happen from time to time. There are three things you can do to help minimize aggression among your molly fish:
- Avoid overcrowding the tank
- Provide lots of hiding spots
- Keep more female than male molly fish; a ratio of three females to one male is best