Water params look fine (0
ammonia, 0
nitrite, 5
nitrate), all other fish in the tank are beautiful and healthy. None of the fish (including the angel) showed any signs of distress or disease. Got home yesterday and found her stuck to the filter intake

Nothing looked out of place when I examined her--healthy coat, no scales missing, no nipped fins. The other angel is doing perfectly fine, as are all other fish in the tank (a dwarf pleco, a brown loach, 2 red shrimp, 2 platys, and 2 zebra danios). I feed on an alternating schedule (angels like the flakes and nothing else, so they get it Monday Wednesday and Saturday, all other fish get sprinkles/flakes on Tuesday, Friday and Sunday, every Tuesday and Friday I drop in two tablets of algae for the pleco and shrimp, in case the algae in the tank runs low). Water hardness and and all other factors look fine to me (checked with
API liquid kit). Angels were aclimated over the course of two hours with small pump cycling the water in the plastic bag with the water in the tank (pump ran REALLY slow...like a couple drops per second). Over the course of 3 hours it's supposed to completely replace the water in a half gallon bottle). Just trying to find out what happened to the angel and how to avoid it in the future...
Bit of note about the angel though: it was a somewhat beat up angel from the
LFS that they had in care. One of its...whiskers? On the lower side was half nipped. LFS assured me (they deal only in fish, they are great people and very knowledgable) that this wouldn't affect the angel in any adverse way--she will swim slightly off balance at times). Aside from that she was completely fine.