What can I add into my Water Flosser?

Mouthwash for flavoring, chlorhexidine if prescribed by your dentist. 

Dr. Frey's recommendation: Add a capful of Listerine Whitening with warm water to your water flosser to help reverse gingivitis. Because of it's antibacterial properties, hydrogen peroxide releases oxygen that helps destroy bacteria which may help battle periodontal disease.