EVERYTHING BUT THE INTERNAL ISSUE is irrelevant at the moment, if other fish are not also struggling, the water quality is clearly fine. Is the fish eating? If not, an injected treatment might be necessary. A veterinarian might be a good person to contact asap to see if they can get you the meds to do this. If they are eating, you need to investigate what the worm is that you are currently facing, and feed a medication that kills it. It looks as though it is currently passing some dead worms, so they may already be dead, but now your fish needs to pass a big pile of worms that were likely imbedded in its digestive tract, and are now starting to decay inside of the poor fish. To aid this, you need to attempt to reduce the risk of infection by giving antibiotics as someone mentioned above. I would not stress the fish anymore than you need to at this point, I would say avoid cleaning the tank or adjusting parameters until you see it pass more of the worms. If other fish are just fine, then the environment isn't the big source of stress, and changing it just adds stress.