When a fish suddenly becomes sluggish and has a loss of appetite its a signal of a possible intestinal bacterial or parasitic infection. If possible remove the Angel and put him in a hospital tank. If no tank is available it's important to keep the display tank extra clean in order to ensure the other fish don't get sick. It would be a good idea to make a 30-40% WC
asap making sure to do a thorough gravel vac. That alone might help your Angel. Watch your Angel for white spots or fungus. I'm not your go to guy for med. advice. I treat my fish with natural methods. Keep updating. If your Angel doen' t improve, there are a few AA members that are very knowledgeable about meds. And i' m sure one will chime in.