at night, when the lights are off, photosynthesis isn't occurring therefore, disolved carbon dioxide which is usually being removed by the process of photosynthesis, is building up in your tank as fish and other creatures respire. it probably happens every night and you just haven't noticed it before. After your lights have been on for a couple hours, it will probably correct itself. You could try to get your fuge lights to come on when your tank lights turn off so that photosyntheses is occurring in you tank durring the day and in your fuge at night. or an easier method would be to make sure you have adequate water movement at the top of the tank. The surface tension of water doesn't allow for much gas exchange. So if you shoot a powerhead at the water surface, you breake the surface tension and allow the Carbon Doxide to be released.
