Was away for a few days came home and my flowerhorn/green terror mix had this on him. He's not eating and it looks horrible. His purple rose queen mate seems just fine.
Looks like the issue Calibigrob had on his pike, which he used an epsom salt to treat it. You can ask him the dosage he used in the 40g tank.
calibigrob
Yea that thsng does look like what my pike had What i did was use 40gallon n added 1/8 teaspoon per 5 gallons of water. Did this every other day ,with 40% water change My pike wasnt eating as well but never got weak Took him maybe 3 weeks cure it This him today U can see that its all gone
how is he fish doing? it looks like a mass or more likely infection/ruptured abscess. Can you put him in his own tank? Can kind of change how we treat him