This might seem oversimplified, but is there anything in the tank that looks like the object? My guess would be some kind of pellet food that just got lodged in the gills, or maybe fish poop that it tried to eat?
Was thinking this too but can't think of anything like this in the tank. Sometimes there are hard lumps in the flake food though, maybe it got lodged. Fish is still alive as of now, does not have any appetite though.