Quote:
Little is known about Isshin's history, except that he was once a captain-level Shinigami, though his division is currently unknown. Twenty years before the main storyline, he ceased being a Shinigami, although it is unclear whether he officially quit his duties at that time or on an earlier date. His current Shinigami uniform retains a captain's robe, but it has no visible division insignia and is worn like a sash over his left shoulder.
and he did use his sword against Grand Fisher btw. it just seemed like he didnt. its known the size of a zanpaktou doesn't really matter (lol XD) and can easily cut something much bigger if they want to and isshin seems to controll that power very well if u look how he cut down that arancar:p