AQ salt will do the trick if he does indeed have fin rot. Also, you can get Stress Guard to help them calm down and get faster regrowth on their tails. How many water changes do you do a week and how much? Moving them all the time can easily stress them out too. Also, try to get a timer for the light; bright lights for too long of a time will also cause tail biting. You can set the timer to 4 hours in the morning and 4 hours in the evening.