In all reality, Maracyn 1 and 2 are not very efficient antibiotics for a number of reasons. Neither work very well in waters that have high pH and Maracyn 2 becomes highly toxic if used after the expiration date. Both have been used so much that most bacteria have actually built up a resistance to them.
Most of the bacteria that we tend to deal with in the aquarium are gram negative, with the exception of mycobacterium and streptococcus. Further, the bacteria that are most commonly associated with fin rot are all gram negative being aeronomas, pseudomonas, and flexibacter. In the instances of flexibacter, triple sulfa has shown to be a very effective medication.
EDIT: That looks like fin biting with the beginnings of an infection.