Try this, seriously.
Get a small hand mirror, the sort of things you get in ladies handbags. Stick it onto the front of the tank, facing inwards and sit and watch.
I had a really aggressive Odessa barb at one time and when I did exactly that, he took umbrage at the other competitor he suddenly saw. He did his best to attack the image and was really put out when the fish he saw suddenly came for him at the same time he went for it.
It took a couple of days but he calmed down quite a bit from then. I think he gave way in hierarchy to the image in the mirror and became docile, even hiding away from it.