Are you talking about Bettas? I am supposing so since you posted in the Betta Board. If you had two male Bettas together then you are very lucky that they are not dead. All you can do is treat the injuries of the one who is not doing well and hope he survives. If he has injuries you will need to keep him in very clean water and in a cycled tank with Melafix and Pimafix in it. Those are herbal remedies that will fight off finrot and bacterial infections in the injuries. They are also mild enough that they will not cause him to be immune to other medications should he need them later in his life. He will need to be kept warm (about 82 degrees if possible - 28 degrees C) and have an airstone to keep the oxygen level of the water up.
Please let us know how he is doing and keep in mind that we have seen Bettas with terrible injuries recover. They are tough little fish and if given the tender loving care and attention they need they do have a good chance of recovering. Just take a moment to breathe and realize that this is a learning experience and you are not the only person to have done this. It is not something to beat yourself up over. Just go on and take care of the fish and love them and let them love you back.
If I can be of further assistance please let me know. Welcome to the Fishlore family and may your experience be as happy and great as mine has been.