Some ammonia test kits I've found will give false low level positive readings. I haven't used that particular one, so would recomend testing it against some known ammonia free water such as fresh RO water. As for recomendations on brands of test kits, I'm partial to Salifert's kits. They are the closest thing to what I used to do in Marine Chem Labs in College without being too complicated for the average hobbyiest and very reliable and accurate to boot
I've found Selcon to be much better than Zoe and Zoecon. It will cause the skimmer to stop foam production after adding to the tank, but it usually returns to normal within a couple of hours.
As ATJ told you, carbon has been implicated by some sources. I'd advise sticking to good quality carbons. One of the best I've seen the US market is Marinelands Black Diamond. It rinses very clean, and the fine dust particles common to most cheap particles is one of the things I've seen mentioned as possible problem with carbon in relation to HLLE.
With the red areas on the tang, I would first verify that test kit reading for ammonia. If it is indeed reading accurately, take steps to improve biological filtration and get that reading to zero. Next, get that selcon and the carrot shavings into the diet. Odds are good that with improved water quality and diet that the red will go away on it's own in this case