graphite disease

I'm not sure if this is his coloring, fin rot or mycobacteria. I got him from petco 2 days ago and I'm keeping him in a 3 gallon tank but I would like to move him into a 10 gallon tank but after reading about mycobacteria and what it does I'm a little afraid to add him to a tank with plants

About 5 days ago I brought home a mustard gas betta. Since I've gotten him his tail is half black, but it's always been like that and isn't spreading. People over at reddit have been screaming for me to chock my tank full of meds, but I really want to make sure because I know some fish...