Please help my Severum I don't know what's wrong

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My Severum is usually king of the tank which is why it seemed odd yesterday that she wasn't going after anyone, and today I came inside to her laying on the ground. She just lays there with the odd attempt at swimming but she is very weak. Loss of appetite. It looks like her stomach is kind of dark and splotchy and it looks like one half of her face is swollen and discolored on the cheek. Everyone else seems fine. There's a little white thing coming off on of the fins but I thought that was just damaged from one of the other fish. I also just did a water change yesterday

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Upon closer inspection I also see an indentation just above the Gill plate and the fins look a little tattered

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