Basically you need to change it out that frequently because carbon can only absorb so much while cleaning your water. In a SW
system the carbon is going to remove bad things along with good elements in the water. AS a result of this it gets "used up" very quickly in a SW
Also something to think about is GAC
can leach phosphates out into your system and cause problems. LR
and a good skimmer is what is needed for filtration in a SW
system. The only time I use carbon is when I need to for a specific reason and it is for a specific amount of time.