I can't find the article, but they found that the more irregular shaped ones were much better than the pellets- which makes sense as they will have more surface area. I use marineland. It was second to another brand, but 2nd out of about 10, so pretty darn good. The brand that was #1 was way more expensive. A lot of the uber expensive ones that were pelleted did not perform nearly as well. This was for removing coloration. Sorry I can't find the article, I've spent 5 mins looking. I should really bookmark these!
-A