How long has the tank been set up?
What are the exact water parameters?
Any white salt like granules on the bodies of the fish?
Have you added any new fish recently without quarantining them?
How often are they flashing against objects in the tank?
Some flashing is normal, especially so after a water change.
If you have flashing without any outward signs of a problem, water quality is always suspect.
Does the flashing seem to be focused on the gill area?