As I mentioned earlier, because your tank is young.
I suspect that it is still undergoing part of the cycle, the addition of live plants depending on when you added them will help a lot.
Disease is a general indication that your water parameters might be off, fish stores want your money and might say your water is good, well good to them, is not always good to the fish.
Continue the treatment now the carbon is out the filter. It will help a lot as well not too feed too much during the treatment as excess waste will cause potential issues with your tank.
You MUST however, with nothing taking particulate waste from your water, perform water changes and vacuum to prevent decaying food / waste from making matters worse.