Why does my Swordtail fish chase my silver shark all the time?

My Swordtail fish is always chasing around the silvershark fish that i have in my tank. I asked the guy at the aquarium i bought them from if they would get along, and he said yes. However, since i put the silver shark in the tank (the swordtail had been in there for 2 weeks alone previously) the swordtail has pretty much chased the silver shark around the tank. Can anyone explain why this is and what i can do about it? Thanks

probably just trying to play/annoy or defend territory.