Your first course of action should have been to move the fish IMMEDIATELY to another tank making sure the tank is also cycled. "Borrow" established filter media from the main tanks to instantly cycle the spare tank. In absence of spare tanks, use a large bucket or rubbermaid tub. Be sure to keep the container well aerated.
Your second course of action is use sodium chloride. Aquarium salt, kosher salt, rock salt or pickling salt will work. AVOID anything containing ferrous cyanide or yellow prussiate. Iodized salt is fine. Add a teaspoon per gallon and be sure to dissolve it first. Add another set after 12 hours to reach 0.2% or 2 teaspoons per gallon total. Should be effective enough to keep bacteria from latchinng on the wound.
For ulcerated wounds or sores, swab with iodine solution (betadine) or hydrogen peroxide. HP should be used only ONCE. Add Biobandage, neosporin or polysporin on it afterwards.
Lastly, water changes, water changes, water changes. Clean water is very important for deep wounds. Do NOT forget to redose salt every time you change your water. Keep a logbook of the dosage if you cannot keep track of it.
