Hi all,
While it is true that the tank water in aquaria with actively growing plants may be oxygen saturated during the photoperiod, when light levels fall below the plant compensation point oxygen usage during respiration will exceed the production of O2 from photosynthesis, and the plants will become part of the oxygen utilising bioload. During the dark period the amount of oxygen utilised by the plants and tank is dependent upon a number of factors (surface area, whether plants are emergent or submerged, chemical oxygen demand, substrate etc ) and it maybe that oxygen depletion does occur in warmer water and effect fish with a higher oxygen demand, particularly rheophilic species from cooler environments (Hill stream Loaches, Chaetosoma etc.).
CO2 is a factor too, particularly for those who leave there's on at night, fish showing symptoms of respiratory distress are unlikely to be suffering directly from low oxygen levels, but are more likely to be stressed by high levels of carbon dioxide in the water. Without a sharp gradient between CO2 levels in the blood and CO2 levels in the water, it becomes increasingly difficult for the CO2 in the fishes blood to diffuse out across their gill surfaces into the water (due to the "Bohr effect").
Having said that I've never seen any signs of oxygen depletion in my tanks including one with Otocinclus in it where I turned the filter of to clean it before going on holiday, and failed to turn it back on before I left. When I came back 10 days later fish were still fine, although some surface scum had developed.
cheers Darrel