There are many purported causes for HLLE, many of which are antipodal . HLLE is really more a syndrome, likely resulting from a deficient immune system. So, keeping that in mind, you just are not going to get "this is absolutely the reason" causing HLLE. Therefore, its trail and error to figure out exactly what is causing HLLE in your particular fish.
While there is nothing wrong with carbon per say, particularly in a FO system, carbon filters will retard the effectiveness of natural filters (live rock and live sand fauna) in your system. For instance, in the case of carbon filters, the food source for filter feeders are demolished by this type of filter, eliminating the growth and development of better, natural, more efficient filter feeding organisms. When at all possible, it is best to move your system in the direction of using natural, rather than mechanical or chemical, filters. Carbon filters, anyway, unless changed often, like once a week, do not meet their full potential and can end up catching debris resulting in added unprocessed waste that will cause nitrate buildup.
As I said, polishing the water with a carbon filter occasionally is fine but a move in the direction of making your system self-sufficient is absolutely the best way to go.
