Have you checked your water quality? There are several reasons for fin and tail rot. It is a bacterial infection and can be brought on by stress in addition to poor water quality or stress from poor water quality. If there is ammonia in the water, this could cause him to breathe hard. I would do a 50% water change every day for 3 or 4 days and watch him closely to see if his fins improve. If not, an antibiotic is in order. The products Melafix and Pimafix are natural and can be very helpful in treating fin and tail rot. You can use them together and be safe. But, water quality is the key here. You could also try a salt dip which consists of 4 tablespoons per gallon for about 15 to 20 minutes. Be sure to watch him and if he should pass out, immediately put him back into the aquarium. Ideally though, he should be in a quarantine tank regardless of the technique you choose to use. Good luck.