Join Date: Sep 2011
Location: Mesa, Arizona, USA, Earth
Plants use CO2 during the day in photosynthesis, which is the process of turning it, in the presence of light and water, into sugar for energy. The by product, or "waste" of this process, is oxygen. Like (most of) all living things, plants respire ("breathe"), which is the process of using oxygen to break down complex sugars for usable energy. The waste product here is CO2.
Respiration occurs 24 hours a day, regardless of whether the plant is in light or darkness, and is always using oxygen. Photosynthesis only occurs in the light. Normally the plant uptakes much less oxygen than it produces durning the day, resulting in pearling and a net increase in the amount of oxygen in the environment (and happy fish). At night photosysnthesis stops, but respiration continues. Now the plant is using oxygen just like your fish, but not releasing any into the water. It's also not uptaking carbon dioxide, but in fact releasing it into the water. In very heavily planted tanks with a high fish load, this can result in an oxygen shortage in the morning. You can then have fish fighting for breath in poorly oxygenated water.
Hope that makes sense!