I haven’t treated canker, but it is supposed to smell bad. Perhaps it could be wet pox or a yeast infection (candida.) Can you take her to a vet or have a farm vet take a sample of the gunk? If you want to treat for canker, look for Fish Zole online—it is metronidazole and available in several stores.
https://www.jefferspet.com/products/fish-zole-metronidazole-tablets-250
Petsmart/Petco sell one brand called API General Cure that has metronidazole:
https://www.google.com/shopping/product/14729732276691146700?lsf=seller:8064,store:12395729436163259491&prds=oid:13859396706112159031&q=api+general+cure&hl=en-us&ei=RKQqW7viO8Py5gL31JHwCA&lsft=utm_source:google,utm_term:5112949,utm_medium:cpc,utm_content:GSC+-+High+-+KOI/MAP+-+Specialty++|+Fish,utm_campaign:GSC+-+High+-+KOI/MAP+-+Specialty&utm_medium=cpc&lsft=gclid:CjwKCAjw9qfZBRA5EiwAiq0Abe-wHaHjOuVKgdVmIAqQ9nJYSWwYaj2aFV7n1hoBcE0JRgQGNL7gzBoCgMwQAvD_BwE
https://www.jefferspet.com/products/fish-zole-metronidazole-tablets-250
Petsmart/Petco sell one brand called API General Cure that has metronidazole:
https://www.google.com/shopping/product/14729732276691146700?lsf=seller:8064,store:12395729436163259491&prds=oid:13859396706112159031&q=api+general+cure&hl=en-us&ei=RKQqW7viO8Py5gL31JHwCA&lsft=utm_source:google,utm_term:5112949,utm_medium:cpc,utm_content:GSC+-+High+-+KOI/MAP+-+Specialty++|+Fish,utm_campaign:GSC+-+High+-+KOI/MAP+-+Specialty&utm_medium=cpc&lsft=gclid:CjwKCAjw9qfZBRA5EiwAiq0Abe-wHaHjOuVKgdVmIAqQ9nJYSWwYaj2aFV7n1hoBcE0JRgQGNL7gzBoCgMwQAvD_BwE