What does it mean when fish swim to the top of their tank and start opening closing mouth super fast?

Fish swimming to the top of their tank and rapidly opening and closing their mouths could be a sign of various issues:

1. Oxygen Deprivation: If the water in the tank lacks sufficient dissolved oxygen, fish may come to the surface to gulp air at the water's surface, causing them to open and close their mouths rapidly.

2. High Ammonia or Nitrite Levels: High levels of ammonia or nitrite in the water can cause fish to experience respiratory distress, leading them to swim near the surface and exhibit increased mouth movements.

3. Gill Problems: If the fish's gills are damaged or infected, it can affect their ability to breathe efficiently. This may cause them to swim to the top and exhibit rapid mouth movements in an attempt to compensate for reduced gill function.

4. Stress: Sudden changes in water quality, introduction of new fish, or aggressive tank mates can cause stress in fish. Stressed fish may swim erratically, including near the surface, and show signs of mouth movements.

5. Disease: Some diseases can affect the respiratory system of fish, causing them to gasp for air at the water's surface. If this behavior is accompanied by other symptoms, such as lethargy or changes in appearance, it could be a sign of an underlying illness.

6. Temperature or pH Imbalance: Extreme temperatures or significant pH changes outside of the fish's ideal range can lead to respiratory distress and surface behavior.

It's important to observe your fish closely and assess the overall tank environment to identify the potential causes and take appropriate measures to address them. Providing adequate oxygenation, maintaining proper water quality, and addressing any underlying health concerns are crucial for the well-being of your fish. If the problem persists or you suspect a disease, consult with a veterinarian specializing in aquatic animals for further diagnosis and treatment.