There is no such thing as "internal" parasites. Other than worms. They are protozoans and they are killed by heat and salt and chemical dyes. (Salt does not have a significant effect on Hardness or PH.)
Fungus/ Parasites http://www.pond-life.me.uk/fishhealth/fungal.php
Treatment: Methylene Blue and or Malachite green + heat and Salt (Sea salt not aquarium salt which lacks many Minerals)
as many call it can actualy be one of several bacterium, Flexibacter, Pusedomonas, aerononas, Fish Columnaris | Fungus & Saprolegnia | Treatment & Prevention
Treatment: Terramycin, Kanamycin, Minocycline
My favorite is terramycin... TERRAMYCIN 200 FOR FISH (OXYTETRACYCLINE QUARTENARY SALT) POWDER [PHIBRO ANIMAL HEALTH]
Of course with all antibiotic medicines, it is more effective if given 3x daily via feed. Skin absorbtion is less effective than going directly into the fish!
Fish can also get Gram positive infections such as dropsy, they can get tumors, and they can get virus. Best thing to do is keep a QT tank for new shipped or bought fish, or at least a hospital tank. Methylene blue is safe and can be used to sterilize nets and siphons, Or more approiately sterilize new fish so they don't get sick or the rest of your tank sick. Don't add methylene blue to a established tank because it will destroy all bacteria good and bad.