Might be a good idea to just add 2 or 3 more Angels into your 55 gal
. If you go that route, buy one Angel significantly larger than the current dominant one, and one smaller than the current weaker Angel.
Cichlids often bully each other to death. If the subordinate Angel is to that point, sure, transfer it over to the reserve tank to heal up and grow a bit.
A solitary cichlid in a tank often becomes even more intolerant towards new introductions. Point being, upon reintroducing the healed Angel back into the 55 gal
. Likely the same situation will arise again unless the Angel population is increased.
Cichlids develops a pecking order. As it sits right now A = dominate Angel, B = subordinate Angel. There is nothing between them to dilute A' s aggression towards B.