Looks like it's much more cost effective to purchase activated carbon in bulk and bag it yourself than buying the pre-packaged carbon. Any brands better than others? What are some good bags to use? This is for a Fluval C2 HOB. Package of three is only $8.99, so dirt cheap, but if I'm replacing the carbon every 2 to 4 weeks as Fluval suggests (of course they do), then it could get pricey.