I have a 225 gallon aquarium. I have 10 large fish inside. The most aggressive fish is my Red Devil. He normally chases all other fish and the others will rarely if ever attempt to go after him. This has changed in the last week. He is now very shy, stays in the upper corner of the tank or hides behind the rocks. He is also eating very little. Nothing has changed in the environment. I have had the same 10 fish in the tank with nothing added and no deaths for the last 9 months. The setup itself is about 4 years running now.
I did notice a small gash between his eyes. I am not sure if what is inside the gash is an infection or just the way the tissue is healing. It does not look like it's full of pus or anything. But it's like white/green in color. Could anyone shed any light on why this fish has had such a major change in behavior? What can be done to get him back to his regular self?