My cobra pike has been doing this for 6-7 months now since she was only a little skinny 4" girl. She is now about 8-9 inches and super fat. She was kept previously with rays who flourished and I know water quality is not the issue. I would assume any type of internal parasite would have killed her by now. It almost seems like a territorial thing. Sometimes when my channa is poking his head out of his decoration and looks like he wants to come out the pike will go over to the opening and duck her side down and rub right next to his opening. I would do what the others have advised and good luck...