Many people will notice a difference when they do and do not use carbon. The idea is that the carbon is removing beneficial trace elements. This is usually most apparent in reef and planted tanks but only when trace elements are the limiting factor. Most people are not at the point where trace elements are their limiting factor.
There have been a few cases where carbon use actually caused HLLE. It is rare, but one of the things to look into when a fish with HLLE has it regardless of water quality and diet. I have talked to a few people who had not gotten any improvement in their fish with HLLE after improving water and food quality. When they removed the carbon the HLLE went away, when they returned it the carbon came back. So they ditched the carbon completely and the HLLE went away permanently. The idea is the same, that it is removing vital trace elements. The reason may not be understood, but the cause in these few cases was obvious.
The most important thing for any product is to see how well it does in your tank. If your tank seems to do better with carbon, use it. If you are thinking about trying a new product then get one bottle/package and go through it completely. If you see an improvement when you start using it then keep using it. If you see a decline in anything when you stop, keep using it (this would mean that the improvement was not obvious, but was there). Obviously if you see any decline when you start using a product stop immediately (and try to get a refund).
