The Sick Fish are a group of sick fishes who have dropsy that are never healed again.
When Milo pretended to be sick, he had to go to the Dropsy Colony and thought that there was going to be people "just like him", he ended up going through a tube leading him in a tank surrounded by sick fish who also have a case of dropsy. He looked around seeing sick fish's coughing, making Milo think they put him in the wrong tank. At the tank next to them, Coach Salmons was throwing a recovery party for Albert Glass. Milo, who is now pretending not to be sick, asks Oscar if he could go to their party, but Oscar doesn't let him for lying to him and everyone else and making him give him the ice cream he inteded for Albert. A sick fish then gives Milo a casserole and his skin peels off, making Milo freak out ("Dropsy!").