Can you post some photos of his fins? This will help people determine the extent of the damage and/or infection.
I would start by doing a series of four 25% water changes, waiting about 15 minutes between each. This will help remove any toxins or bacteria in the water. (Water changes never hurt, and frequently help!)
Fin rot is usually caused by gram-negative bacteria. Copper-based medications are not needed for issues like fin rot.....
If you decide to treat him with something, I would start by putting him into a separate quarantine or hospital tank so that your catfish won't be subjected to any treatments or medications.
From what I've read, Firemouth Cichlids can tolerate a small amount of Aquarium salt. Salt has some antimicrobial properties, so this may be a way to treat an infection holistically. (Source: "Firemouth Cichlids can tolerate up a salinity of about 10% of a normal saltwater tank or with a specific gravity of less than 1.0002." Firemouth Cichlid, Thorichthys meeki Fish Guide, Cichlasoma meeki
If the fin rot has set in already from an infection that occurred this morning, however, this can indicate an aggressive infection. In this case, you may want to try using a true medication. (However, I'd like to see some photos before making any specific medication recommendations.)
Again, I'd start with a series of partial water changes to help remove bacteria and toxins from the water. Then, post photos so that people can see exactly what's going on.