Red pest disease or pond pest as it is also known is caused by a bacterial infection (bacterium cyprinicida).
The disease causes blood red patches to appear on the body of the goldfish.
These can be hard to detect on red goldfish or dark colored ones such as Black Moor. The only indication on these fish is an increase in body mucus and clamped fins.
There is only one cause for this disease, poor water conditions causing the fish to be weakened enough for the bacteria to take hold.
This disease doesn’t appear to be infectious to other fish; it seems to be an opportunist disease that attacks weakened fish. It will still indicate that the water conditions aren’t good, and it will only be a matter of time before other fish will succumb to the conditions that are causing the outbreak.
Immediately make a 50% water change in the aquarium or pond, and check if the filter isn’t clogged. Also check for anything dead that could be polluting the water. Fish like to hide when they are sick and then die where they can't be easily seen.
If nothing obvious is found, check the pH and ammonia levels.
This disease is one of the easier ones to treat.
Move the goldfish into the sickbay and add five teaspoons of non-iodized salt per gallon.
As this is an external bacterial infection, I also add any of the normal bacterial medications such as Methylene Blue.
Malachite Green which is a synthetic dye can also be used but isn't as safe as Methylene Blue.
Keep the fish in the sickbay until all traces of the disease are gone then slowly reduce the medication at each water change. Feed only live food.
The aquarium or pond will require a complete clean out before the fish is put back.