[DiverDude]

I change once a month. However, I think it might vary depending on if you use carbon in a reactor or in a bag.

I just have a bag hanging in my sump and my thinking is that rate of flow through it is lower so once a month is fine. If I had a reactor, I might rethink that.

I haven't entirely convinced myself this thinking makes sense yet though !

Interestingly enough, if you read what the carbon people say, they say to only use carbon intermittently in a marine tank. I think 99%+ of people here run it full time.