For carbon to work best it comes down to the same old issue there is with any media, including UV and that is dwell time. We usually uses filters for double and tripple duy. First we use them to clean the water using various media and bacteria. But we also use them to create current and often to agitate the surface to promote gas exchange. Howevr, when it comes to letting the bacteria do its thing, how long the water is in contact with the media before exiting the filter (or a UV unit) is important. This is especially true for carbon. Fast flow is an enemy of effective use of carbon. With A UV most folks have and independent circulatory method that is slower flow than the filters on the system. But for many of us carbon gets put into the same filter that holds everything else and which also creates current and surface agitation, This makes the carbon less effective.
Ideally, one should have a separate filter for the carbon, one which has a slower flow rate. it can still host bacteria and/or mechanical media, but it wont produse a lot of flow. For many this is not practica,l so we work with what we have.
As an aside, I run one tank where I want heavy staining and have to work to keep it that way. I am adding catappas, alder cones and even peat plus brewed rooibos tea. I rotate old wood out for new now and then as well. I have to work to keep it this way- just the opposite of what you are looking at.
Ain't fish keeping grand?