i use sprint fixer and a standing water bath between my ansco 130 and fixer
and i have never gotten dark prints ...
maybe PE is right, try it with running water ... or make a dilute stop bath and see if still happens

i use running water AFTER my fixer, a water bath tray that i drilled holes through so water
can escape, i have a siphon hose but i don't use it for my tween dev/fix bath