It looks like an injury to me but I am not 100% certain. That being said, if it is an injury it can easily become infected if the water levels are too high. 1ppm ammonia is much too high and we like to see nitrate levels between 5-20. If it were mine I would do 4 25% water changes 4 hours apart to avoid shock. You can get some indian almond leaf for the tank to help with healing. Are there sharp bits on his plants or décor?