That cotton looking stuff on your fish is a secondary fungal infection. If black spots are preceding on your other fish as well, Its very possible/ probable that it is ammonia burn.
Here is my suggestion:
First adding copper to your pond will forever ruin any chances of snails and other natural beneficiary to your pond its toxic for these little guys.
Adding PRIME available in your local pet store is a good way (however not a permanent solution ) to decrease ammonia in your tank. Use double the dose (mentioned on the bottle)
The wool/cotton looking fungus is a sign of poor quality water regardless of what the pet store says, filtration is key here. Here is how to fix it; Quarantine fish that are affected. A hospital aquarium is best here, small enough to keep the fish comfortable without having to use massive amounts of medicine to dose large amounts of water. Remove any carbon filters on your hospital tanks. Use Melefix and Pimafix ( I suggest adding to your pond if all your fish are affected) double the dose of API aquarium salt along with E.M Erythromycin. Change about 50-60% of the water in the tank every day. The key here is super clean water. Before using Erythromycin use melefix/pimaflix and salt. These are natural and best ways to cure, if the problem persists feel free to use Erythromycin. Keep posting updates, ill keep an eye on your thread.
Quick Questions I have for you:
Is feces from your fish, white?
What filtration are you using?
How long has your pond been established?