I have had 6 zebra danios for a week now, the first few days were absolutely fine. A few days ago the thermometer popped off the side of the tank and was floating around in the water. The biggest danio was circling it and aggressively chased any other danio that came near it. I put the thermometer back and all was fine again. Then I added a fake floating jellyfish into the tank, the same fish did the same thing again so I took the jellyfish out. I have now noticed that the same danio won't let the others anywhere near one whole half of the tank. As soon as any of them get to the half way mark he chases them over to the other side, but strangely only when the tank light is on. As soon as I turn the light off it stops immediately. What can I do? Surely I can't just never put the light on?