Catfish death

This morning I found that one of my two pictus catfish had died. William and I went out and bought two more catfish of different varieties. When I got home with them, I found that the remaining catfish was behaving in a very distressed way, swimming upright facing the surface, gasping for air, gills pumping hard and beginning to turn red. I got the second air pump going, replaced about a third of the water, and added all other helpful chemicals I own. It is sitting on the bottom now, I think too exhausted to swim like it was doing. I think it is also dying. Have not yet found anything on the internet to help.