Probablly due to the size of the tank im guessing. Most people would suggest a 75 gal is good for a single oscar, however you hav 2 plus a jack dempsey. I think the oscars are just getting a bit rowdy because they are cramped. Also you might have a breeding pair of oscars? I have no idea, they might want to get rid of the jack dempsey? How big are the oscars? I would remove the jack and just keep your favorite of the two oscars. At least remove the JD...hope this helps...