Update on the tank. The day I put the meds in I saw one of my pygmy cories flash against the substrate just once, haven't seen it since but am glad I did go down the medication route as there's clearly something bothering the other fish, even if it's not doing so routinely. The main panda (the one who lost his barbels) is feeding again and his breathing seems less laboured although not 100% normal. I'm hoping his barbels will grow back?
I'm hoping that this has done the trick. I haven't seen the "main" panda for a couple of days but couldn't find any evidence of death when I cleaned the tank this morning so I'm hoping he will turn up! I haven't seen any flashing by any of the other fish so keeping everything crossed that this has worked and that everyone has come through unscathed.