Indeed. There are tetras, and then there are tetras such as Buenos Aires'. Too boisterous for a discus tank. Will annoy the heck out of the discus, successfully outcompete them for food, swim around fast, and result in stress. You may be able to get away with it for a bit, but not for long. Discus need to and will establish their own pecking order. Generally there are 2 extremes of results, with usually nothing in between: either full success, or sickly-looking discus who will darken, cower and eventually die one by one, according to pecking order.