All I can say is I hope that is a 150G + tank!!
Tiger barbs are mean, but not likely to be the one. You should return it if you only have 1. They are mean and will pick on the others if not in a group and will also get sick more easily. Post in the trade area if you want to just get it a good home. Send me a picture if you're in the MD area.
And Blue Gouramis are also VERY aggressive fish. I've seen them kill other fish, but with that many. Their attention should be kept to them self‚Äôs.
Rule out the Plecos if they are under 8 inches. But you should return 3 of them. 1 is more then enough for most tanks up to 120G.
Danios attack something...that would be a first
Harequin Tetras...again.....a first
the most likely is the Red Tail. If it saw thre sword tail and mistook it for one of it's own it would attack. They are very territorial. That or anther Sword tail.
Honestly it might have been sick or gotten a parasite.
I count at the lest 70 inches of fish. I really hope this is a massive tank. 55G is not massive.