Sorry for your loss, if something like this happens, I do a couple heavy WTC.
I also sometimes feed some anty parasite food from jungle labs, if I do not see agression. In most cases that is what it is, you might name a couple pairing up.
Glad everyone else is doing well. Watch them close. I agree with DavidZ....whenever I see someone "off" in one of my tanks...I start doing BIG water changes. Sometimes daily. Until they show me something else.
Sorry for your loss. I have kept Angels for a lot of years & I always shed a few tears when I lose one.
I lost one this week as well due to unknown reasons. I had had it for about 4 months, they were all fine- this one started hanging around the corner & not eating. No signs of disease- parameters fine. I thought maybe it was being bullied as the larger Angel is showing some agressive signs. Found it dead yesterday am. Other 2 seem fine.