I think you'll find it a hard search to find a vet who knows anything about fish. The people here, especially those who have been in the hobby for a number of years would be able to give you very sound advice and i would trust them over any vet when it comes to fish. As dimebag said, what type of fish are you treating?
Is Droncit pure prazi? If so many websites would be able to give you a few different, equally as reliable, ways of treatment.