Ulceration disease (Achlya) is caused by a fungoid infection.
The disease causes filminess of the eyes and swollen gills.
It must be treated swiftly otherwise the goldfish will rapidly die due to breathing difficulties.
There is only one cause for this disease, poor water conditions that have caused the fish to be weakened enough for the fungus to take hold.
The water conditions must be extremely bad for this disease to appear. If one fish has succumbed, others will follow shortly after.
Change all the aquarium or pond water immediately.
The treatment is the same as for fungus.
My preferred method of treatment is a combination of a salt bath with Methylene blue added.
I gradually increase the salt content from two to 5-6 heaped teaspoons per gallon. I then add Methylene blue until the water is medium blue. Goldfish can tolerate quite high doses of these chemicals safely. The cure is slower, but safer.
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.
Because fungoid diseases are always present in still water, sterilizing the aquarium or pond may seem pointless. But, whatever caused the outbreak needs to be removed from the aquatic environment. A water change may not be enough, especially if there are large amounts of rotting debris sitting on the bottom of the aquarium or pool.