you've already started the full-spectrum treatment, but before that i would have suggested, since the damage appears to be localized, or at least locally presented, to try something more focused, such as putting neosporin on the reddened area. it likely would not hurt the condition, but it may have helped it heal up without the need for broad spectrum antibios.
you could still try this treatment with the fish now, it would just be tougher to figure out which treatment is working. polys are definitely a peculiar bunch when it comes to ailments IMO. i have had success with neosporin on localized damage/infection in the past though. good luck, and i'm sure the bichir will pull through!--
--solomon