03-13-2011, 06:16 PM
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Originally Posted by DKRST
During darkness, plants respire by taking in oxygen from their environment for cellular respiration. If a tank is very heavily planted with minimal surface agitation, the oxygen levels can drop significantly during nighttime hours and CO2 levels will increase. The statement that oxygen can never be low in a planted tank is not, biologically, quite accurate. If you found your fish at the surface in the early morning, before the lights are on, I'd recommend turning on some additional agitation at nighttime to increase the oxygen levels and, just as importantly(!), drive off excess CO2. Having said this, it's does not seem to be a typical problem for folks who don't use pressurized CO2.
I did qualify my statement with the words "provided all this is in balance" though perhaps it should have been explained more to be clear. Provided there is a biological equilibrium in the tank, with fish, plants, bacteria in balance, and with a natural method (no CO2 addition) there will never be an oxygen shortage that is detrimental to fish, plants or bacteria. The plants produce far more oxygen during photosynthesis than the fish and bacteria will consume during daylight; and during darkness the majority of fish are resting and thus consuming very minimal oxygen compared to daylight. With this correction, I agree with the rest cited.