first you need to realize what carbon is mainly used for, and its to remove additives from the tank. like after a medication treatment. outside of that i would always go for more bio-media. carbon is most often added to cartirdges because it needs to be replaced often. its a great cash cow. it does work, to an extent but its not necessary. i know people who have had filters for like 2 years and havent replaced any media in it besides some rinses in tank water.

goldfish dont need salt. the other thing is if you were adding salt per recommendation but only 10% waterchanges your salinity probably was getting up there. you only re-add salt for the amount of water removed and added, not redose the whole tank. additionally out side of live bearers salt doesnt really do much for freshwater fish outside of treating for ick or something.