Pond fish with fungus
I suggest isolating that fish in its own pond or tank salting it at 2 TEAspoons of salt per 1 gallon of water (that's 5 pounds of salt for 150 gallons) 1 time only, for a maximum of 10 days.
In addition, add quick cure (we sell that here) 2 drops per 1 gallon (or 1 ounce per 270 gallons) repeat daily for 7 days. Turn UV off while using Quick Cure.
The same cure will also kill ick, but with ick, you have to treat the entire pond, because Ick is water born.
Fungus comes from a wound that gets infected, so normally un-damaged fish will not get fungus.