The active ingredient in Marycn is erythromycin. Erythromycin is in an antibiotic class called the macrolides which are generally gram positive spectrum. It's got to be kept up at certain levels as it's mechanism of action slows bacteria down, reversibly damages some cellular machinery, but gives the fishes immune system a chance to catch up. The key is it needs to be used against the right kind of bacteria. Erythromycin is mostly used in aquariums to clear red slime algae, which is also known as cyanobacteria(though i feel there are many bhetter ways to deal with cyano). It is one of the anitibiotics that doesn't mess with your nitrifying bacteria and it leaves your cycle alone. I've used it with success to save fish that had bilateral popeye or dropsy and assume that these are systemic gram positive bacterial infections causing problems with osmoregulation because of either liver or kidney damage. I haven't seen anything on this this in scientific literature for ornamental fish; mostly salmonids. In my personal experience it's worked where gram negative antibiotics failed on butterflyfish, anthias, soldierfish, and an angel for those symptoms. I haven't had it work well for cloudy,sloughing, or red external problems of fish. For that stick with maracyn 2 which is minocycline, a tetracycline that targets a broader spectrum of gram positive and negative bacteria. I like furan for those external things as well. If you can get an aminoglycoside like kanamycin (kanacyn), gentamycin, or amikacin, I prefer those for all external lesions as these are bacteriocidal and don;t need to be kept at a concentration...you spike them every day or two and they do permanent damage to bacterial cellular machinery.