Clown Died

My ocellaris clownfish (aquacultured) died today. I got it 2 days ago from Petco. All of my parameters were perfect. My PH was a little low but i used Kent Marine Superbuffer to raise it. At first, he was the only clown but seemed a little lonely and was getting bullied from the other 2 damsels in the tank so i bought another one. The other one is still alive and am hoping the same doesn't:confused: happen to him. The clown that died was fading in color and was very limp. He was breathing heavy and was not moving, he was just floating around and then moved when he ran into something. He then eventually died not much later. He is now starting to get dark on the upper part of his body. He had white stuff coming out of the bottom of his body but im assuming thats normal feces. But i could be wrong. I am new to this and have no idea what went wrong. Help!