Welcome to BYC. Most people treat canker with metronidazole 250 mg tablets daily for 5 days., also called Flagyl and Petsmart sells it for fish under the name General Cure. These are online from many fish stores too. Can you post a picture of the canker? We love pictures.
What dose were you using? Canker can look like wet fowl pox, and last week someone posted a picture that looked like canker, and a few days later the lesion fell off. So posting your picture may help--I'm certainly no expert, and have never treated it, but many others on here can usually help you tell what it is because they have seen it.
You probably know more about it than I do then. Are you treating with the 250 mg per day dose of metronidazole? Acidified copper sulfate in the water once a month has been used by some on here to control the illness in a canker positive flock. Pictures are helpful, however, to those of us who have not seen it.
The Tylan powder I have says 2 teaspoons (tsp) to 1 gallon of water.
MG = Mycoplasma gallisepticum
