Porcupine Puffer just sits at bottom

My porcupine puffer just started sitting at the bottom of my tank. I've had him for about a month and he just started. Or he sits in the top corner of the tank. He is housed with a yellow tang. I have noticed that the yellow tang does flip his fins at him. Also I seen him puff up today. It wasn't a full puff but it lasted for about 5 seconds. Any help would be appreciated!