This is very accelarated and should have been treated Day One. We are past that so lets attack this-
Is this fish eating? if so - Good. We are going to boost its' immune system by adding garlic extract and selcon vitamins to its food,
IF NOT, , , , , , begin with a freshwater dip for 4-5 mins ( this will sort of cleanse the flesh and rid of any flukes IF present. This bacteria which has attacked tissue is likely associated with water quality.
Any ammonia/nitrate readings you can provide?? Last time you did a water change?
Salinity should be 1.024-1.025 and Temperature at 78-79.
This is very similar to Vibriosis which is a type of internal infection caused by a genus of gram-negative bacteria called Vibrio. This disease is commonly contracted through contact with open sores or dead fish and, once a fish is infected, the disease tends to progress quickly. Because it is largely an internal infection, many fish suffering from the disease do not show any external symptoms until the final stages of the disease.
Some external symptoms associated with this disease include red streaks on the body (an indication of internal hemorrhaging), red spots, dark swollen lesions and sometimes cloudy eyes. Behavioral changes may also recur as a result of this disease which might include lethargy, loss of appetite and difficulty breathing. The most effective treatment for this disease is oral antibiotics such as kanamycin – this type of treatment is best administered in a quarantine environment because anti-bacterial meds could damage the biological filter in your tank.
if you dont have Kanamyacin, you can use melafix but this is temporary, not the required med.