Actually that white spots from the onate you have on the thread that you shared are slim coat loss, if you look at the picture most likely its due to the common pleco in the same than as the ornate, common pleco or some pleco likes to suck on polys slime coat, so i dont think its the same problem your having.I don't think it's costia but you definitely got me on the right track, I think I have a diagnosis now. I looked up "bichir costia" and the first thread got me to here: https://www.monsterfishkeepers.com/...es-bleeding-pics-disease-bruise.551781/page-3
I guess this is why we quarantine. Bichir worms... Sounds pretty nasty.