Why does my sinus smell fishy in my breathe.

Halitosis: Hello, the smell you are noticing may be coming from a sinus infection, post nasal drainage, or possibly from tonsil stones. Increase your fluid intake, gargle with water after meals, and rinse your nose with salt water twice daily. If this doesn't help, see your doctor.

Sinus infection: Bad breath can be due to a sinus infection, gingivitis, dental carries / abscesses, gastroeophageal reflux disease, zenker's diverticulum or esophageal diverticulum. Depending on your symptoms and on physical exams, your doctor can narrow down a diagnosis and give you proper treatment. If you have previous history of sinusitis, it might be the cause of your bad breath now.