I need help deciding whether to euthanize a fish, I've never seen an injury this severe and I have no clue if this fish can recover or if it is having a long lingering death. This is a very tiny fish, a Long Finned Leopard Danio completely healthy and fully finned but for this horrible injury. First off no IDEA how this happened I am really upset at myself that it even happened (can angel fish do this?!), it wasn't there at 7am feeding but at 9:30am I checked in again and the little girl was mangled. She is shredded top to bottom from just behind her side fin all along her side to roughly parallel her anus, it's complete hamburger meat. Her fins are fully splayed and she is clearly in distress swimming in small shimmying circles, if she were a dog she'd be limping in circles. I did searches and everyone is saying fish can recover from an amazing amount of damage, but to my newbie eyes this looks insane. If she can be saved can someone walk me through the process of helping her or point me towards a post about treating fishes with severe injuries. I tried to stock up on anything my fish might need so I have Pimafix, Melafix, Aquarium Salt, Prime, Florish Excel, and lots of other stuff since I figured if I needed it I would need it immediately. Water is good and tested weekly and is consistent results (0 Ammonia, 0 Nitrate, 0 Nitrite) except for the PH which is a little low at around 6.3 to 6.5 currently it keeps dropping from a 7 to a 6.