I worry about giving antibiotics on a conjectured basis, as other problems, more commonly found in ducks such as an aspergillus, can be greatly worsened when antibiotics are given by eliminating any good bacteria, allowing fungi to colonize at a faster rate.
Respiratory distress in waterfowl is predominantly related to tracheal obstruction, which can be due to fungal spores, tramua, granulomas, seeds, or other matter. Conditions such as EYP, or salpingitis that cause increased pressure on the lungs can result in dyspnea. However, determining which one might be the cause is hard over the internet, but nearly impossible without a video, or further clarification on the birds past history, and symptoms.