Hi there sorry to hear about the fish. We keep fancy goldfish. How many gallons is the pond? Could you post pics of the fish? Fin rot causes red streaks in tail and can lead to death, white spot looks like some one sprinkled salt on fish, cotton fungus is as it sounds. Unfortunately the list is endless without pics. Melafix and primafix are the best on the market and can be given together. All natural stuff, solve parasites, worms, fungual infections, fin rot, bacterial infections. Most of the fish fanciers that show and breed use these two. You can buy mega bottles of the stuff from specialist shops or internet.
Afraid that quarantine would have prevented it, but easy to say with hindsight.