[DiverDude]

I emailed Kent and got a timely reply from their lead tech guy !

He says:

"It actually depends on the type of a marine system. If you have just a
basic Marine tank with only fish then it isn't as big of a deal. If you
are keeping invertebrates then you only use carbon intermittently. Carbon
will pull out some essential trace minerals and other components of the
water that inverts require. These would be things such as Iodide,
organics, some metals, vitamins, etc. It is recommended to use carbon only
if you need to clarify the water or pull out medication, etc."

This would suggest that those who are running carbon full time are paying for the carbon and then paying to add trace elements that it removes. Of course it's also removing other toxins (if present) as well so that may be worth it itself.

A dilemma !