Why does my silver tipped shark just spin in a circle for hours on end?

I have a 30 g. aquarium, and my two silver tipped sharks, appx. 6" and 5" long respectively, act so.. erratic. I've had them for about 8 months, and they are always so HYPER. Lately, the larger, "lead shark" has taken it upon himself to swim in a vertical circle at one end of the tank... Is there a reason for these sharks neurotic behavior? I'm about to move them to a 40 g aquarium so they have more room to "frolic."