Hiya and welcome to AquariumAdvice rikiv1!
First and foremost, what are your water parameters (ammonia nitrite nitrates)? Is always the first thing to check when one is having problems with one's fish. What size tank is everyone in? Whats the temps? And how long has it been cycled?
Secondly, I'm guessing your didn't QT the new guy. I strongly suggest getting a cheap 10g, filter and heater and making sure all your new guys spend 2 weeks in it so you can make sure they are healthy. My guess is the new guy came from the
lfs with something, although we need to know those parameters to be sure.
In the meantime, it would be a good idea (if you DO have a QT tank) to remove him from the main tank and put him in there. This way, if you have to treat with meds, it won't mess up your nitrifying bacterial colonies. Plus, he will be less stressed im a quiet dark area by himself.
Besides the obvious weakness, are you noticing anything else about the new angel? Heavy breathing? Any strange marks or growths? Blood in the fins or under the skin on the body? When did the weakness start (just today? Or was he getting progressively weaker)? What are you attempting to feed him?