They could be spooked, did the fish just come out of a fully planted tank with gravel and other fish?
Sometimes when you move fish into bare bottomed tanks they freak out I think the gravel gives them security, you can add some more hiding places for them to make them feel more secure or even a thin layer of gravel, or even some smaller more confident dither fish (that wont fit in their mouths) or they might just settle in and get over themselves..
Good luck!