Found this on a site hope it helps
Many people also ask about torn fins. First, will they grow back? It depends on how damaged the veins or supporting structure of the fins are. If the support structures (like the veins of a leaf) are gone, the fin will not grow back fully. If they are intact, it may grow back completely. Either way, the fins will probably never look like they used to look. Even if the fin is totally gone, it may grow back partially, but will probably look abnormal. Fins are often ripped by other fish or injury or sometimes from an infection. So the second question is how do you treat torn fins? If the fins show no signs of fungus or bacteria, then try salt and MelaFix. If the fins have fungus, use a fungicide. If there is an infection, use antibiotics. Some antibiotics will kill good bacteria so be careful. Erythromycin such as found in Maracyn by Mardel does not harm the good bacteria. Salt treatment should be about a tablespoon per 2-5 gallons (I use a tablespoon per 5 gallons at all times). Use only pure sodium chloride like that sold for aquariums or ponds or marine salts. Do not use salt with iodine such as table salt. Salt will stave off infection and help prevent electrolyte loss to the water. Also, MelaFix by Aquarium Pharmaceuticals may be added which contains tea tree extract. It is safe for everything but will cause some foaming on the surface and smells a little bit. Tea tree is a natural antibiotic and antifungal. A little aloe added to the water may also help aid healing. Or, add something like Stress-Coat which contains aloe. It is also a dechlorinator. After a fin is injured, when it is properly healing, the edges often turn black. This is normal and means the wounds are healing as expected.