Keeping 2 dwarf angels in the same tank is usually a recipe for trouble. In a large enough tank, where each fish can claim its own territory, it may be possible, but otherwise the fish will not normally get along.
I find for adding new fish, its best to place the new fish in my isolation chamber. The isolation chamber is an enclosure made from eggcrate & zap-strapped together. It allows my new fish to see my old fish & vice versa. At first the old fish will try to attack the newbie through the chamber. After a week, they seem to ignore each other & its then safe to release the newbie.
How's your stocking rate? Fish breathing heavily are usually a sign of high stress & low dissolved oxygen. The stress may be as simple as the new fish disturbing the tank's equilibrium. It may also be the new fish introduced something bad into the tank, such as parasites or gill flukes. Your observations will be needed to diagnose the problem.
Sorry for your loss.
Anthony
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