i can't make a definitive diagnosis from your picture. i'd use salt as follows, just in case.
2 teaspoons per actual gallon of water.
to add salt, mix it with some of your tank water in small volumes and add to your tank. never dump salt in directly as a solid. add ¼ tsp. per gallon once an hour for four hours. this will bring your tank to a level of 1 tsp. per gallon in four hours. then increase it to 2 tsp. per gallon in ¼ teaspoon increments every 3-4 hours. always watch the fish closely for reaction. when you reach a level above 1 tsp. per gallon, watch for signs of increased stress and if noted, slow things down and allow the fish more time to adjust.
carbon will not remove salt from the water so if you are using it you do not need to remove it as you do with meds. table salt, pickling or canning salt, kosher salt are all fine to use. the levels of iodide or flow agents are too minute to be a factor.
during this treatment, increase the tank temperature to at least 82°F. important note ... warmer water does not carry oxygen as well as cool water. increase oxygen concentrations in your tank during treatment by (a) lowering the tank water level to increase splash, (b) raising the spray bars above the water line, or (c) add an air pump to your system with a good bubbler.
maintain this treatment for two weeks minimum.
it's important to continue a water change regimen during treatment and i'd recommend twice a week ... but remember to replace what you remove. just add whatever your target level is to the change water. for example, if you have 2 tsp. of salt per gallon in the tank, and you change 10 gallons of water then add 20 teaspoons to the change water as it goes back in. the total level of salt in the tank will remain the same.
