When I first set up my tank I had purchased 3 tetras for cycling, they sat in a corner near their reflections shaking for a week,not eating at all until 1 of them became a victim of my betta and the other 2 carried on with this behavior refusing to eat and acting strung out so I went and purchased 4 more and the change in their behavior was instant,I fed them about ten minutes after adding the new fish (which they instantly found and befriended) and they ate for the 1st time in two weeks, so maybe your sharks just lonely or feeling intimidated, I guess it happens,but read about it before adding more because I've heard that if there's only 2 of a certain fish sometimes the biggest will pester the smaller and make them miserable so it's better to have them in groups of 3 or more