what kind of test are you using? how long was the tank up and running? how long did you qt the new fish for? what kind of substrate? what are the parameters of the tank? what is the temp?
it sounds like they were really stressed if they were up at the top gasping for air. im guessin an ammonia spike. if you have strong filter then it might have pulled it pretty fast out of the tank but not fast enought to keep from harming the fish. just need more info on the tank and paramters to get to the bottom of it. i would let the tank fill in the way you want it and then add new fish. also with the tank planted like that it will stress angels some cause they need room to swim. if you want a "jungle" feel i would plant the vals along the sides and across the back kinda in a U shape that will still give lots of swimming room and you can always add something like dwarf sag to the tank if you want more of a cover on the grown