Possible Reasons:
* Territoriality: Mollies can be territorial, especially when it comes to breeding or feeding. The goldfish may be occupying space or resources the molly considers its own.
* Competition: If food is scarce, the molly might be competing with the goldfish for food.
* Stress: Overcrowding, poor water quality, or a lack of hiding places can cause stress and aggression in fish.
* Breeding Behavior: If the molly is female and pregnant, she might be acting more aggressively to protect her eggs.
* Disease or Injury: The molly might be acting aggressively due to an underlying illness or injury.
What to do:
1. Observe: Carefully watch the tank to see if there's a specific behavior that triggers the aggression.
2. Assess Tank Size: Make sure the tank is large enough for all the fish. Goldfish need ample space, especially as they grow.
3. Water Quality: Check water parameters (ammonia, nitrite, nitrate, pH) regularly and ensure they are within safe ranges. Poor water quality can trigger stress.
4. Hiding Places: Provide ample hiding spots for all fish, especially the goldfish. This can help reduce stress and give them a place to escape.
5. Feeding: Feed the fish multiple small meals throughout the day, ensuring all fish have access to food.
6. Tank Mates: If possible, consider separating the molly and goldfish. They may not be compatible tank mates in the long run.
7. Disease: If you suspect illness, consult a veterinarian or knowledgeable fish specialist.
Important Note: While it's possible the molly is just being a bit feisty, her behavior could escalate if not addressed. Always be watchful and take appropriate steps to ensure the well-being of all your fish.