When you get more plants before putting them in remove the aggressive angel and place it temporarily in a container. Totally rearrange your tank and then add the angel back in. This often helps alot as it tricks the aggressor into thinking it's in a new tank where it doesn't have a set territory or the tank is mine syndrone. And every once in a while you just get a nasty tempered angel. It doesn't happen often in juvies but it can. If it continues you have a couple choice.... first you could rehome the aggressor and try another angel or you could add 2 more angels to distribute aggression out more. But in your size tank you would possibly need to rehome 2 as they mature and get large depending on their temperments.