Make sure the quarantine tank has a lot of aeration. I had a betta with severe persistent case of fin rot and not one single medicine worked on him until I put an airstone in his tank. He's doing fine now. He was rehomed to a larger 5 gallon tank with serious filtration in the form of an AquaClear filter and a bubble wand for aeration. I will never ever keep a betta in anything less again, having seen for myself the importance of oxygenation and filtration. His previous tank was 2.5 gallons and had a small Red Sea brand filter that had just one little block of floss in it.
I have to add that I had performed "chemical" surgery on him, removing the damaged fin parts by smoothing them off with my fingers and betadyne solution (very caustic to fish--use only on afflicted dead tissue). But you can't do this with a cory. That would stress a cory to death. My betta isn't bothered by anything--the most easygoing friendly fish I've ever seen. Still, the edges retained some rot until I improved his living conditions as stated.
P.S.--what is the pH of your water? On the planetcatfish data on Sterba, if I remember correctly if the pH falls into the low range, their fins can start to erode. You may want to look this up. Their site isn't coming up for me at present so I am unable to provide a link for you. Also...there was some discussion about air-using fish with the labyrinth organ not being able to tolerate one of those mela or pima meds. I'm sorry, I don't recall which one it was.