No problem. Just tell the RCMP that you're bringing in a potential fertilizer explosive across the border to help a friend run a "plant growing operation".
I'm sure they'll be REAL understanding, especially after you set off all of the bomb-sniffing dogs.
That being said, KNO3 is working great for my crypts, swords, and hornwort, the plants consume any ammonia as soon as the fish add it so they appreciate the nitrogen.
Alternative idea: You can get potassium legally with potassium sulfate or potash from the home improvement store garden department. Maybe stock some more fish to get more nitrogen?