A fluffy fungus attacked one of my goldfish. Apparently, the goldfish already has a problem because it is lemon yellow. Can you help me? Will it spread to the frogs, koi or other goldfish. I don't want to kill my fish.

Do you have salt in your pond? If not, you can add about a 0.05% salt solution to the pond without harming any animals or plants. It will help deter fungus. If you can, you can treat the fish out of the pond with higher levels of salt as well as anti-fungal medications. Fungus is always in the water and just takes advantage of a fish when it has an injury so the other animals should be okay if they aren't physically damaged. Antifungal medications can be added directly to a pond but that would be expensive and could harm the non-fish animals. Frogs, koi, and goldfish can all get fungal infections but usually only when something else is wrong first.

I have added salt. MY lemon yellow goldy has died. How much salt for 19 gallons?
I don't want another goldfish to die. Another one has caught the fungus.

Is this a 19 gallon pond or aquarium? That's hard to maintain for a pond. For a 0.05% salt solution in 19 gallons of water, you would add just an ounce of salt or so (0.07 pounds). If you don't have plants, double it for a better effect. Since the pond is so tiny, you can add fungus medication like MarOxy by Mardel without much monetary cost. It may harm invertebrates though. A bigger pond would help the goldfish fight off things like fungus.

I'm upset. I've had the yellow goldy for 20 years now!

I'm sorry he died. You had him for a good long time though and can remember him fondly I'm sure.