Hello,
Organic carbon is the third nutrient. It is often ignored because we as hobbyist can't measure it. Relatively new very expensive equipment is available for scientific research and has been used to gather data on organic carbon levels in the sea,surveyed reef tanks and aquariums as well as it's effects on corals. In brief, it feeds non photosynthetic bacteria, and in excess it is thought to upset the corals holibont bacteria creating pathogenic activity.
Here is one article with a good amount of detail and references in it:
http://www.advancedaquarist.com/2008/8/aafeature3
I don't think there is a practical way to tell the levels
of organic carbon in our tanks. But certainly adding more via a carbon source will raise it without attention to export via skimming and even more so granulated activated carbon. This is why I think the pellets could be good if they do in fact keep the carbon out of the tank as opposed to direct tank dosing methods such as sugar for example.. Monitoring orp drops might give some indication that it is building up but not with any precision or reliability, in my opinion.
I can't explain why organic carbon in water would not be taken up by bacteria. Perhaps the observation was inaccurate or influenced by some unknown external variable or the actual pellets used were not biodegradeable( ie had no organic carbon in them).