Hmmm... I never put more then 1/4 tablet in mine. And as far as the repeating goes, keep adding more with water changes until you see a definate improvement (clearish regrowth and whatnot). Don't keep him on it for a long, long time, but keep medicating him till there's no sign of rot left. If you go off the meds and the rot retuns again, start the meds up again. If you find the maracyn isn't helping or doing its job, then I'd suggest moving onto something higher, like tetracyclene (if you can find it) or perhps the furan-2, also kaynacyn, as Super suggested, works well. All three are broad-range anti-biotics that cure both gram -negative adn gram-positive bacteria.
Good luck, and don't worry - I'm sure he'll get better soon!