Hi Ufishy! Maracyn contains erythromycin, which is effective against gram-positive bacteria. Tetracycline and ampicillin both treat mostly gram-positive bacteria and some gram-negative ones.
Fin rot is caused by gram-negative bacteria. There are several medications that target that type specifically. One good one is Maracyn-Two, which contains minocycline. Another good one is nitrofurazone, which is found in Furan 2 and Jungle Fungus Clear (even though the name says "fungus," the ingredients are antibacterial).
Here's a link to a page discussing various antioiotics and whether they're for gram-positive or gram-negative: Aquarium Medications and treatments; Antibiotics/ Antimicrobials