Water changes. Daily, enough to keep the ammonia at less than 1 ppm. This will help ease the stress on the fish considerably. If it's already to the point of septicemia, the tang might not make it. They tend to be pretty sensitive to stressors, and the ammonia, as well as the smaller tank, will cause problems. Removing the tank to quarantine (with daily, 50-75% water changes) and treating with a broad spectrum anti-biotic might help--but this can't be done in the main tank as it will kill what beneficial bacteria are present.
How long has the tank been setup?