Being as you only have a 45g tank you could get those nudibranchs....just an option.
You do know that a powder blue in such a small tank is gonna be trouble right? That right there is your issue with the aggression between the copperband and him. Both fish need a much larger tank, the tank substanially so.
180g Reef - 60g Rimless Reef -90G FOWLR- 125g Malawi Cichlids- 40b REEF- 34g Fresh Planted-working on- 20L FOWLR- working on
I have a mix of many different saltwater fish amongst my tanks, but I love my Tangs most of all.