well when I had a large Mbu puffer in my 1,000 gallon not too long ago.. he would get so scared he puffed up. So I knew he was stressed out.. but now that he is with my friend in his big tank, I don't have to worry about him.
my fishes get scared to sometimes, since when I siphon the tanks, they would be swimming around. Sometimes they do get scared.. and sometimes they don't. I have some rare angels, so I have to do a
water change every 2 days. Keeping the tank very clean.
So my fishes usually eat the scraps of food floating around on siphon day, but now that they are somewhat used to it, I don't have to worry much.
I'm sure your Angel just needs some time to get used to the big siphon.
good luck!