Yes, you can but there are better medications available, such as Kanamyacin. From the reading I have done, very few of the problems are from gram positive bacteria. The other problem with Maracyn II is that it is a teracycline family drug which means it is inhibited by calcium. If you have hard water it will inhibit it, so you need to add more. The only antibiotic that I have found to be effective is Kanamyacin.
Join a fish club. Don't have one in your area? Start one.