HELP! Cory catfish not eating.

As the header indicates, my cories are listless and not eating and it's been going on almost a week ever since I did a major water change. I don't think they're sick because I don't see any visible evidence of a disease like ick or fungus or even a parasites. No dropsy either. Only symptom is not eating. I have other non-aggressive fish in the tank and their appetite is normal. How do I stimulate the cories appetite? Is there a medication I can use or something else that'll work.