You don't have to use the Fluval carbon media. I have found that any carbon media works fine.
However, there is one brand I found in a warehouse aquarium store that works GREAT. The brand is obscure, but it something like Virgin Carbon "The one that pops!" (the say it pops because it makes snapping sounds when you put water on it) I know it sounds silly but I swear by this brand because it keeps my RES tank sparkling clear. If you can't find that particular brand, just look for one that has little dust in it and just make sure you rinse it well before you put it in your filter. Hope this helps a little bit at least!
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1 Dog ~Charmin~
1 RES ~Loquat~