carbon
 

Should i be running activated carbon in my canister filter on a reef tank? i am converting my fish only to a reef. i have heard yes and no.

It is good to run a small amount of carbon in a filter sock placed in an area of high flow in your sump. Running large amounts of carbon in something like a canister filter will provide lots of surface area for aerobic bacteria to grow which can increase the nitrates in your tank. Small amounts of carbon in a reef tank should be more so to polish the water and remove the yellow tint that can occur.

how often should i change it out?

Every 2-4 weeks should be good. It is better to use small amount and change more frequently as opposed to large amounts being changed less frequently.