Just looks like physical damage , rubbed or bumped up against something trying to squeeze into a hide that's jus a bit too small isn't uncommon for bichirs lol.
I'm sure the question about stocking was to see if you had a pleco they can suck on bichirs, heater burn is another good guess but ruled that out as well. The only other thing you might want to check is your filter intakes, I've seen round patches like this from suction against a filter intake after knocking the guard off. It's all the same basic ideal though, physical damage to that area and not an infection.
Clean water and time and it'll clear up, a little salt wont hurt, scrap the med's it's pointless ad likely going to cause unnecessary stress.
Thanks for the thought. Deep down I think I knew it was probably some damage from rubbing up against some driftwood or something, but was unsure if it was something more serious due to fear of losing the sen. It's been acting healthy this whole time so I'll probably take it out of QT today.
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