i know i'm late coming to this thread, and i'm very sorry for your losses. at the first sign of any kind of trouble, the first thing i do is dose the tank with salt 1 tsp. for every 5 gallons and raise the temp to 86 degrees and add at least one more air stone or bubble wall. do a 25% water change every day and redose the salt for the changed water.
IF after 7 days this doesn't work, buy a bottle of pimafix and follow the dosing instructions on the bottle.
the ONLY time this hasn't worked for me (no matter what was wrong with the fish) is when i bought my altum angels. 4 actually did survive and grow to adulthood, but i lost more than 10 of them. and i found out after the fact that i should have treated them immediately on arrival with Furan-2. but the altums didn't have ich or any kind of fungus.
i've heard many horror stories like yours from folks that use the "treatments" that they sell at the LFS that say they cure Ich. I've heard a lot of folks kill catfish by the dozens using copper-safe. and yet folks still buy all those bottles of chemicals, and some folks even tell other people to use them. i don't know why. i DO know why the LFS' sell them, it's great for repeat business.
about carbon and filters.. carbon is NOT necessary, it will help keep your tank looking sparkly and crisp, but it has little other purpose. and good filtration will do that just as well. there are some theories about extended use of carbon causing HITH disease in cichlids, especially oscars, but i think it's just a theory. either way, i see no good reason to waste money on it.
IF you are choosing to use carbon, no matter how long it's been in the filter, you should remove the carbon only, not the mechanical filtration media, when you dose the tank with any meds, even salt.
i'm sorry this is too late to help the fish you've lost. but now you have real good info in case it happens again.