A stuck rock would not explain the heavynes ?If u suspect a small rock is stuck inside the spiracle. I would wrap the bichir with a wet cloth. Inspect the area and get a tweezer. If no gravel present. It may be some sort of infection if the membrane area on the spiracle is swollen.
He doesn't look like swallen and i have Black ghost knive in the same tank so salt could harm it?. Maybe a little epsomsalt and just trying to get it to eat? that might be tricky though.hopefully he hasn't swallowed any of those rocks and is suffering from a blockage. maybe add some salt help him poo???
Is clown knife a scaleless like bgk?I wouldn't feed him just incase its a blockage. not sure if salt would hurt your knife. I have a clown knife and I use salt with no issues.