Maracyn contains Erythromycin, Maracyn Two contains Minocycline. Two very different meds for different types of bacteria. One antibiotic attacks gram positive bacteria and the other, gram negative. Knowing the type of bacteria you are battling comes in handy when choosing one over the other. If you donââ"šÂ¬Ã¢"žÂ¢t know the kind of bacteria you are dealing with, then you need to go by the fact that most bacterial infections in marine fish are GRAM NEGATIVE bacteria. Thus, Maracyn Two for Saltwater fishes would be the medication of choice. The great thing about this medication is that it will work even when your fish will not eat. It penetrates into the fish and even helps the fish overcome internal bacterial infections (like those that cause eye swelling and/or Popeye conditions).
Maracyn One for Saltwater fish is an antibiotic that attacks GRAM POSITIVE bacteria, with B complex vitamins. This will have a distinct negative impact on the biological filter (as do most GRAM POSITIVE antibiotics).
Seeing that the highest probability is that your fish has a gram negative bacterial infection, the choice would be Maracyn Two for saltwater fish. Maracyn One doesn't hurt, but as written above, you can pretty much count on your biological filter to be wiped out. This might be unnecessary if it wasn't the one to use anyway.
