happened to mine too, male bit off a sizable length of my females tail.
it took forever to heal up, i noticed it healed more quickly when she was sectioned off from him, than when with him, possibly continued nipping at it.
to heal her, i just used a little salt with water changes and kept the water pristine and fresh.
i did try melafix and it didn't seem to do diddly squat other than making the water smell nice and foamy. the key is keeping the water clean, making sure they eat (keep their energy up and helps the body heal quicker) and helping them feel less stressed (sectioned off).