I have been treating my 7.5gal tank with Melafix because one of my guppies had finrot. Unfortunately he died, but the others are ok. I took the carbon out of the filter while treating. Should I put the carbon back in the filter now?
Yes, it would be good to put activated carbon in your filter after your tank has been treated. It will help you remove the medication from the tank (together with regular water changes). Besides, a larger water change is good as well after medicating a tank.