HLLE and carbon - astonished! (Long)

[ACBlinky]

I rescued a tang last year, she was in BAD shape with ich and a bacterial infection that had eaten away much of her flesh and fins. After a stay in the QT with meds and six weeks of hypo, she had recovered but still had missing pieces of fin - her dorsal fin in particular looked like it had several large bite marks out of it.

Over time, her fins regrew in all but one place and I resigned myself to the fact that she would forever have a nick in her dorsal fin. Then it started to heal over last month, and I was overjoyed - to me it signified that she was officially healthy; she was fat, seemed to be stress-free, and now her body had enough energy to heal that last injury site.

I hadn't run carbon in a long while, because I ran out and every time I went to the LFS it was the one thing I forgot to get. The tank was slightly yellowish/greenish, and I wanted to really clear the water, so I finally bought some carbon and placed about 2c in the filter sock. I replaced it after a few days, ran some more for about two weeks, and then continued to replace it biweekly for about two months.

Despite no change in water parameters or feeding regime, the tang started showing signs of HLLE. After a month, she had noticeable spots all over her face and one on her lateral line. Her dorsal fin was also starting to back slide - the small nick again turned into a large bite mark. I was beside myself - this beautiful fish was deteriorating, and I didn't know what to do. I considered diet, but it's pretty well-rounded: I feed a blend of frozen seafoods, Formula One and Two flakes, Spirulina flakes, Hikari marine pellets and nori, wakame and dulce seaweeds on a clip daily, and everything is soaked in Selcon.

So I did some reading (thank goodness for RC) and found threads talking about a link between HLLE and carbon. I couldn't understand how something that cleans the water could cause this condition, but it was an easy theory to test. I stopped running carbon right away, and added some Essential Elements, Mb & Se, and iodine (1/4 dose). I normally don't add any of these, they're just left over from last year or the year before, but I wondered if the carbon could have stripped something needed from the water. I didn't do a water change, but that will happen next weekend.

To end a long story, after about two weeks with no carbon she's recovering nicely. Her HLLE is about 1/2 what it was, and her fin has almost completely filled in - it's back to where it was when the problem started, and shows signs of healing fully in the near future.

Has anyone else had this happen? Was it total coincidence?