Any other behaviors, flashing, scratching, like he is trying to shake whatever it is off? Any other fish with similar symptoms? Any recent changes, new additions?
From what I've seen he was just hiding all day not coming out to eat heavy breathing and staying close to the overflow vent when he comes out no new additions
I can't tell a lot from the photos -- perhaps an infection? But my line of sight is quite limited. If it ha ich, flukes, velvet, or brook, it could have turned in to an infection at the insertion sites.
Set up a 5 g qt tank, catch him and give him a 5-10 min freshwater dip to kill most of the disease (make sure u match the ph of freshwater dip), then dose qt tank w praxipro. If he flashes then u could try cupramine. However if it is some type of disease then it will remain in ur dt so u may need to leave it uninhabited for a few weeks to let the disease die out w/o a host.