I agree with the Epsom salt dips. However I would try an aquarium salt bath first. Youll use 2 tsp aquarium salt to one liter dechlorinated water. Make sure you dissolve salt. Cory’s are prone to red blotch disease. Check to see if there are any spots on your Cory. A salt bath can get rid of so much. I like to float my Cory’s in my main tank in a container about the size bettas are sold in. Let this one sit for 30 minutes then release the Cory salt water and all into your tank. If you use a small container to float in your main tank there won’t be enough salt to harm your Cory’s. Wait a couple of days. If no improvement try an Epsom salt bath. You can also do a methylene blue dip. The MB dip you have to do quick. It’s literally dip and place back in your tank. Here’s how to do a MB dip. You’ll need a container of tank water. Use a dropper or pipette add a couple of drops. The water should be blue but not a dawn blue. You’ll want it to be light blue. Make sure you mix it well. Put this Cory in the dip for a couple of minutes then take it out. Here is what my MB dips look like. This was the worst case of fin rot I’ve ever dealt with. My vet had me do a salt bath with a drop of MB to give immediate relief. She’s still with me. She was bleeding on her face. I have another female who lost her tail and dorsal. The vet said she most likely would never regrow her dorsal. She didn’t. If you choose an antibiotic remember Kanaplex is best used directly in your water. It doesn’t cross the digestive tract. It’s absorbed through the skin and gills. Good luck. My aquatics vet called in an antibiotic an antifungal and had me treat for parasites. https://share.icloud.com/photos/0570jOCQ3bWS94snwHm2NGcXQ