How bad is the injury? Is there a wound exposed? Melafix will prevent him from getting most illnesses. Be sure to keep the water quality at its best, and perhaps an air stone will help reduce the stress. Keep in mind this angelfish is using energy to repair its wound(s), so using less energy to retreive oxygen from the surrounding water would help. Please be as descriptive as possible when you let me know about the injury. The more I know the more I can help. Keeping it in a hospital tank was a good move to start. 
Hopefully, that will allow him to be less stressed, and able to recoup.
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Phishie
Site Coordinator
"An ounce of prevention is worth a pound of cure."
