I also use Purigen - in my Eheim. I keep enough for two bags - one in service and one that been regenerated. That way I always have a new bag ready to go into service. Not cheap, but worth it since it can easily be regenerated over and over.
One tip - buy "the bag" (made by Seachem) or the velcro close bags made by Pura. They will hold the Purigen, which is a very, very small resin bead. It will go right through most bags!