Re: Fantail with finrot?
#6
Carbon will reduce the efficiency of Melafix, so it is best removed. I can't think why removing it would affect your water quality unless your bio-filter is a bit dodgy (for example, newly set up) and you need to hold on to all your filter bacteria (carbon will house some filter bacteria, like the other filter materials do).
It's difficult to know about the spot on his belly without seeing it. Any chance you could keep him separate from the other fish for a while, so you can see clearly if this is down to harassment or not? What about a tank divider - simple and quick, and you can get them online.
Consultant, Mars Fishcare.