U never specified that the other dats were definitely nipping at him. The shrimp was the other possible culprit, right..If he's getting beat up, a wide open mouth I think would be uncommon. A floating tank is prob not the best option for a stressed dat. Can you set up an additional tank. If you think he is going downhill quick, it might be the best option. How is he doing? Any sign of rapid breathing? How are the other 2 doing? Are they eating?