Honestly I have never really measured, magnesium sulfate at lower rates is pretty harmless. In a 75 gallon, I would add a couple of cups that have been dissolved in tank water. As far as adding metro to the tank water, that's your call, but if it was my fish at this point I would. Otherwise the chance for reinfection will always be there. The last thing you want to do with meds is under dose, and have any of the pathogens survive. So short of moving your fish to a qt tank, and continuing to treat orally, while treating the display tank on its own, I don't see another option.