Canister filters.

[Myka]

As he ^ said. Although you can also put live rock rubble in the canister. Really it is not very useful in a reef tank at all, more of a pain in the butt to clean than anything.

[Ephraim]

i pack an xp1 full of carbon and gfo. found it to be a better option that most commerically made carbon/phosphate reactors.