So after 4 yrs. powder brown lost the battle. Found it dead this morning.
In the smaller tank the two fish got along just fine but as soon as they were introduced to the large tank they starting bullying each other. And these were the only two fish when I started the large tank back in Oct 2017.
I'm sorry to hear man. After 4 years, its a pet, not just an aquarium inhabitant.
If I had to take a guess, territorial fish can be kept in smaller tanks together because the overcrowding helps stymie their aggressive behavior. Basically, nobody has a territory to defend since they are all packed in there together.
I bet once you gave them a bit more room, one of them decided it had its own established territory and defended aggressively to protect it.
With my stocking, I had the opportunity(we'll call it that since almost all my fish died from velvet) to add fish in the order I wanted and I made sure to put all the aggressive and territorial fish last and together. That way, my tang and angels never had time to establish territories and by the time I put them all in there, it was really too crowded to even have a territory. I get no in-fighting ever with the angels and tang. I do get a some small scuffles with the wrasses but that's it.
My angels are way well behaved. The emperor and blue line keep to themselves which is amazing. 
