03-02-2010, 01:47 PM
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Claudia is correct. Plants and fish both produce CO2 continually and assimilate oxygen continually, day or night there is no change.
Plants consume CO2 during photosynthesis (only occurs in daylight) and produce oxygen, some of which travels down to the roots to be released for bacteria to use in the substrate. And as was said, more oxygen is produced than the plants can use, so it benefits the fish and bacteria.
During darkness the plants rest, and this is crucial, just as it is for fish and all animals, and for exactly the same reasons. Hiscock mentions 10 hours of absolute darkness, and I've not read anyone suggesting differently. I have a lot of fish and plants in my aquaria, and they have 10+ hours of darkness, and I have never noticed the slightest sign of respiration issues that would suggest a deficiency of oxygen in the very early morning. Also bear in mind that during periods of total inactivity (night) much less oxygen is required and correspondingly less CO2 is released than during high activity.
In high-tech aquaria with CO2 diffusion, most recommend turning it off at night to avoid overloading the system and poisoning the fish, and some further recommend turning on an aerator at night; this is because the CO2 diffusion is pumping considerably more CO2 in the aquarium than the plants can probably use (they are very slow at assimilating CO2 as I detail in one of the stickies) and it is more likely to create an imbalance that could be detrimental. But I do not personally believe this is likely in a natural (low-tech) system.
An indication of this is the pH fluctuation that naturally occurs every 24 hours in planted aquaria as it does in nature. During the day, as CO2 is consumed, the pH rises; during night when CO2 is being replenished, the pH drops back. In my aquaria the fluctuation is about 3 or 4 decimal points, say from 6.2 at dawn to 6.5 at dusk. This is not a concern.
Last edited by Byron; 03-02-2010 at 01:50 PM..