okkey there are couple of easy way u can prevent.
1st isolate the fish, kept in a pottasium permanganate solution(1mg per 3 littre) for half an hour,then kept the fish in an isolated place add some salt and set the temperature around 30 deegree............probably help u .because my angel was cured by this method.
2nd ly there are couple of high priced medicine which cure finrot like magic.....go to any pet shop they may assist u