Any ideas on what's stressing out my harlequin rasboras?

You can not just get a new filter and remove the old one. the filter is holding all the god bacteria keeping the tank cycled. The tank has started a new cycle and you will have ammonia and bacteria blooms, if the old filter or cartriges are still wet, put it back in the tank, their is a possobility the bacteria is still alive, but more than likly it is too late, test for ammonia, nitrite and nitrate, do multiple small to medium whater changes unitill ammoni and nirite is at zero and stays at zero.