User blog comment:TVthePunisher/Final Getsuga Tensho and Isshin/@comment-2041622-20100916090050

I dont think Isshin used final getsuga on anyone, the most logical way he lost his powers was because he was in one of Urahara's gigai like the one Rukia was in causing her to become human, well i think that is the only reason Isshin lost his power and to prove my point is that after Isshin killed Grand Fisher Urahara said "so it looks as though your skills are still sharp as ever, what a relief, well i cant have you blaming me if your reiatsu is all shriveled up"