not too much can repair a missing fin.but if there is any remaning filaments of the fin there is a slight chance it may regrow back some fin,but not all.bettamax is a great product and i use it myself.yes i would use this to help heal your betta,but try it at half strength.after such a tramatic injury as a lost fin full strength meds. may be more stressful then helpful.if the betta pulls through the first week or two try going full strenght andfollow the instructions.what kind of other inhabitants are in your tank?i know loches can be very rough but a whole fin thats a mean loach!i have read some studies on fin regeneration and it i possible but there must be some fin remaining to regrow.well good luck this is going to be tough.let me know how it turns out
