From what I have seen Matrix doesnt have a lot of surface area, at I looked at a bottle of matrix the other day and it looked just like crushed gravel like you would use in cement...
Try the "POND" version you'll be happier in an FX since the output is on the bottom..IF you use regular matrix place it in a bag.... matrix is porus you have a lot more surface area than just the outside shell......
DIRECTIONS: For filtration use 2 Liters of Pond Matrix for each 200 gallons of water. Use in any pond filter. Pond Matrix is sufficiently large that a filter bag should not be necessary. Follow the manufacturer's recommendations for your filter. Pond Matrix should never need replacing.
Although they don't have as much surface area, I think people tend to use a ton more media than they need. Scrubbies are probably fine if you aren't using the canister on a ridiculously overstocked tank.