Silver Shark eating Tetras?

After being dumped with all my friends fish because he didn't want them, I haven't been paying them too much attention other than feeding and cleaning like usual. But when I was feeding the other day I noticed a 'husk' of one of the tetras; he must've died and the Silver Shark in there ate him. Then I realized that though I'd started with 12 orted tetras, there were only 6 in the tank. There haven't been any other dead fish in the tank ... has the shark been killing and eating them all? I thought they were pretty pive fish?