Fin rot,Tail rot,and or fungus is indicator that water quality is deteriorating. With bettas,this is usually due to the fish being kept in unfiltered,unheated bowls,vases,etc. In a cycled tank of no less than five gal ,with healthy biological filter, and a heater,there are far fewer problems keeping these fish healthy. The best thing you can do is provide the fish with such an enviornment.