I'm thinking your fish has a secondary infection from whatever parasite you had. I would actually treat this fish with an antibiotic like triple sulpha. If the rot is still aggressively eating the tail.
I just did a big water change and put in a bag of carbon to clean up the meds. Water parameters are still ideal as well. I have to leave town on business for a few days so its going to be lights off with clean water and less commotion around the tank maybe this will help his immune system a bit. If the tail doesn't look any better I will try triple sulfa when I get home. Can the triple sulfa be used with metronidazole if I start dosing with that again?
I would not combine the meds. I would do a complete treatment with triple sulpha and access the fish and then you could treat with metro if need be. Metro is a fairly toxic medication so use sparingly and only when absolutely necessary.
I started him on triple sulfa two days ago. It says two doses is a full cycle then change the water 24hrs after the second dose. If I don't see any healing when can I start a second round of the triple sulfa. I got some methylene blue but i really don't want to remove him from the tank with his tail in tatters.