How much carbon do I use?

How often do I change it?

How do I know when it needs to be changed?

Where is the ideal place to put it?

Should all the water filter through the carbon, or just place a bag where water flows around it?

Any good articles for reading up on?

This is for a 120 gal tank with a 55 gal sump/return/fuge with mostly softies, LPS and a few fish. I'm not in need of a cure for something, just want to add a little extra to help with water quality.

Since I started all this, I've never used carbon.