So the lights go on in my tank and I happen to be there in front of it when they do (they're on a timer). One of my cory cats is swimming frantically at the surface. He's swimming so close that his top fin is actually partly out of the water. I have no idea why he's doing this. I've seen them dart to the surface many times, but he's actually swimming around the tank right below the surface of the water. One of my black skirts is looking at him like he's weird. Is this that normal or should I be concerned?