There are many articles/threads about treating canker here on BYC. FishZole (metronidazole, Flagyl) 250 mg divided into 2 doses 12 hours apart daily for 5 days can be used to treat canker. You will want to make sure you keep the water station clean and sanitized.
http://www.backyardchickens.com/t/773308/canker-treatment (Treatment Post#4, 7, 10)
Photos of canker post #10 https://www.backyardchickens.com/threads/canker.1080266/#10
Give them plain drinking water or add some poultry vitamins to their water a couple of days a week. Clean and sanitize water stations daily.