How much of a water change? Ultimately you should be testing nitrate levels periodically until you get a routine worked out, but I would start with 50% weekly, and increase frequency and/or quantity from there as necessary.
carbon (has been shown to cause HLLE, yes CAUSE),
Do you have any documentation to this effect? I'd be interested in reading that.
it doesnt CAUSE it..it MAY sometimes aid it in some cases..since carbon has the ability to remove necesarry bacteria. i've never experienced it..
The problem with carbon is it removes vital micronutrients and trace elements that are needed for proper physiology. In species susceptible to HLLE (please note I personally refer to HITH as the disease with cases involving parasites and HLLE those that do not involve any parasites) this can end up as HLLE. I believe in many other species it may cause internal problems that we simply cannot see and when they die we blame it on something or nothing (old age, some minor change in the tank, etc.).
I have not seen any scientific studies on this, but I have talked to a few people who have had situations where everything was tried (diet, water quality, treating for parasites, etc.) and nothing worked. Then when they removed the carbon the HLLE went away. In some cases it was returned either thinking they needed to go back to normal or to see if it was the cause and sure enough the HLLE returned. This does not seem to be the usual cause, but it is a cause.
I do not use carbon in any of my tanks because it removes vital trace elements and micronutrients. I simply used the extra trays for Biomax. If adequate water changes are made then no negative effects should be observed (odor, discoloration, etc.).