i was wondering this as well since in the qt sticky post in the disease forum it says that carbon may cause hlle.
Activated Carbon
There is plenty of anecdotal evidence suggesting that the use of activated carbon in filters is a causation of HLLE in fish, particularly tangs. It is purported that HLLE can result when carbon particles leech into the aquaria, or the leaching of organics filtered by carbon, or by the removal of essential water elements by the carbon’s mechanical filtration component. Whatever is the cause, animals prone to HLLE kept in aquaria that relies on natural filters, such as sandbeds, live rock, refugium systems, macroalgae or mangrove tanks and good water movement throughout the system, usually remain healthy and free of HLLE.
That is not to say that hobbyist should throw out their canister filters. Rather, that if HLLE erosion is an issue in your system, or if you plan to keep fish prone to HLLE, then use carbon sparingly rather than constantly. Use it for a few hours occasional to polish the water, not a constant exposure. Natural filters, regardless of disease considerations, is always a healthy choice for captive marine animals.