Same thing happen to my neon tetras. I sometimes think they get shocked when lights are suddenly turned on so what I do, at night before the timer turns the lights off, I turn the switch to the blue light mode. That way in the morning when they turn on automatically is not as bright. Then I change it to white light once they have seen light from the kitchen and living room. I don't know if it makes a difference, but I like to think it does.