Do your fish dart about the tank, crashing against objects within the tank and showing startled behavior?
The inadvertent addition of a toxic chemical can create such behavior. 
So can excess activity near the tank. 
To insure against toxic chemicals in the water, I suggest that you have activated carbon handy and place this within a nylon stocking and place within the filter of the tank.  Make sure that there is a good flow through the carbon.  It will help to remove most chemicals within the water.
If it appears that it may be caused by excess activity, reduce the amount of activity in front of tank or put a towel over front of the tank to shield the fish from the activity.
The first indication of trouble within the tank is the behavior of the fish.  The best time to observe your fish is when you feed them.  Even if you have many tanks, it pays to take a few extra minutes to observe the fish.  It will more than likely spare you time and aggravation later.