Unfortunately when they catch the lil' buggers in the ocean some ppl use cyanide. If they get a good dose the effects can be fatal not just on the short term but months after exposure.
As for activated Carbon it goes without saying that most chemical treatments stick to the carbon like glue as it should. Exceptions being Cooper-safe and sodium treatments. Unfortunately activated carbon is only good for about 30 days - if left unchanged, in time, it will release everything it has been holding back into the water.
I very highly doubt this guy was caught with cyanide. From the day he came to the LFS he was eating and swimming around and active. I had the owner hold him and hee was at the LFS for over 3 weeks. He was healthy and happy and even eating in the acclimation bucket when I brought him home. It was definitely something that happened in my QT.
The only treatments I use are Cupramine and Prazipro. The carbon is never in the tank more than a couple weeks. With the wrasse the carbon was in the tank for 2 days. Something else is going on with it. If it was leaching my copper levels would be higher too, and they aren't. Granted it couldn't leach in 2 days and couldn't make the copper level too high anyway because it was never too high for the last fish in there and before that it was completely cleaned up and empty.