How severe is the fin rot issue?
The easiest way to treat fin rot is to keep the fish in warm, clean water.
If that doesn't work, you can try Aquarium salt. (Do not use this for more than 10 days though!)
Very severe or aggressive fin rot can sometimes be caused be a gram-negative bacterial infection. For this, yes, Maracyn Plus may be effective.
Maracyn Plus contains Sulfadimidine and Trimethoprin. These target both gram-positive and gram-negative bacteria:
"Sulphadimidine is highly effective against streptococcus, staphylococcus and gram-negative pathogens..." (1)
"Trimethoprim is a synthetic antibacterial drug active against a wide variety of aerobic gram-positive and gram-negative... pathogens" (2)