Why would a fish keep swimming directly into the corner of tank if it has been very quickly up ans down for about three days?

It's impossible to say for sure without knowing more about the fish and its environment. However, there are a few possibilities for why a fish might repeatedly swim into the corner of a tank after a period of rapid swimming:

1. Stress and Disorientation:

* Rapid swimming: The fish may have been stressed by something in its environment, causing it to swim frantically. This could be due to a sudden change in water parameters, a new tank mate, or even just a loud noise.

* Disorientation: After a period of extreme activity, the fish might be disoriented and confused. The corner of the tank may seem like a safe haven or a way to orient itself.

2. Health Issues:

* Swim bladder issues: Problems with the swim bladder can cause difficulty with buoyancy control, leading to erratic swimming patterns and potentially a preference for resting on the bottom near the corners.

* Parasites or infections: Some infections or parasites can affect fish behavior, causing them to become disoriented or to exhibit abnormal swimming patterns.

3. Environmental Factors:

* Water quality: Poor water quality can lead to stress and disorientation in fish. Check your water parameters for ammonia, nitrite, nitrates, and pH.

* Tank size and decor: A small tank or a lack of hiding places can cause stress, leading to the fish seeking out a corner for security.

4. Sensory Issues:

* Vision impairment: If the fish has vision problems, it might be having trouble navigating its environment and accidentally bumping into the corner.

* Hearing issues: Fish can experience hearing problems, which could lead to disorientation and difficulty avoiding objects.

What to do:

1. Observe the fish closely: Note any other abnormal behaviors, like loss of appetite, changes in color, or increased respiration.

2. Check water parameters: Make sure the water quality is good and adjust as needed.

3. Reduce stress: Make sure the tank environment is peaceful and that the fish has plenty of hiding places.

4. Consider a quarantine tank: If you suspect an illness, move the fish to a quarantine tank for observation and treatment.

Consult a veterinarian: If you're concerned about your fish's health, it's best to consult a veterinarian who specializes in aquatic animals. They can help diagnose the problem and recommend the appropriate course of action.