yeah fast him for several days and then try pellets, or even worms or something, if he remains super stubborn the insects would be better option for live foods and not have the risk of him getting parasites, which he may already have since it seems like that is pretty much all his previous owners fed him, keep an eye out on his poo and if his HITH refuses to go away with clean water and good diet then i would try treating him for parasites and see if it helps. and i agree with the carbon while it is only suspected i just dont see the need for it, its just taking up space in the filter that could be used for more biomedia, i havent used carbon for the past year and my water is totally clear and i dont have the cost of constantly replacing it. i also just recently rescued an 8in O that somebody brought into petco but he only has minor HITH but had kinda eaten away fins, so im not sure if it was fin rot or what but he was also constipated as well but now just 4 days later he is already starting to look better and has great white growth ridges along his fins where they are growing back, so hopefully they didnt have him for too long and he isnt badly stunted