As the water flow direction in a canister filter, we setup medias follow a rule-- rough> medium rough> fine.
1. Rough media: mechanical filtration medias like filter wool or floss, bonded pads, and sponges
2. Medium rough media: biological filtration medias include ceramic rings, sponges, pads, bio balls and stars, sand, and gravel. One thing to be aware of is that biological filter medias often used as mechanical filtration too.
3. Fine media: a pack of Aquarium Pure is enough to keep your aquarium crystal clear