Pardon me for being busy body.

Take immediate action to see if fin rot is the cause especially the pectorial fin.

I have 4 angel I bought about 4 months ago, all grew very well and attained adult size within 3 months.
As a result of their apparent health, I overlook them and did not take careful look at them regularly.
I now discovered that they all have serious fin rot and the pectorial fin has been so damaged that it has deformed.
So, don't be decieved by the look becuase they may be very healthy dispite their fin being attached by rot.

Just my share of experiece/mistake.

Cheers!

BT Lee